The Holy Spirit is SPIRIT of G-d

1) the Spirit has the divine characteristics- omnipresence (especially: He lives in all believers)
                                       Ps 139:7-12; John 14:17-18
- omniscient                           1 Cor 2:10
- grace                                Zechariah 12:10; Hebr. 10:29
- holiness, glory                      Romans 1:4; 1 Peter 4:14
- life                                 Romans 8:2,10; 2 Cor 3:6
- love                                 Romans 15:30; 2 Tim 1:7
- unsearchable greatness               Isaiah 40:13
- truth                                John 14:17; 15:26: 16:13; 1 John 5:6
- where the Spirit is, there is power  Acts 1:8; 2 Tim 1:7; Romans 15:13,19
- sovereignty                          John 3:8; 1 Cor 12:11



2) the Spirit has divine names

- Spirit of God                        Numbers 24:2; Rom. 8:9,14; 2 Chr. 15:1
- Spirit of the Lord                   Judges 3:10; 6:34; Is. 11:2; 2 Cor 3:17
- Spirit of the Father                 Matth. 10:20
- Spirit of Jesus                      Acts 16:7
- Spirit of Christ                     Romans 8:9
- Spirit of the Son                    Galatians 4:6
- paraclete, i.e. the advocate,        John 14:16,26; 15:26
  helper, counselor 

A Reply for Muslims who want to replace Muhammad instead of Holy spirit

Bible proves that the holy spirit is not a single entity but a human soul. 
Part 3:
Does a spirit means holy ghost, not a human?
No it is completely wrong, a spirit is a human soul in hebrew ruah in arabic ruh.
The proof is here:

Genesis 2:7 (KJV)
And the Lord God formed the man of the dust of the ground, and breathed into his nostrils the bteath of life; and the man become living soul.
Job 33:4 (KJV)
The spirit of God hath made me, and the breath of the almighty hath given me life.
Matthew 12:43
When an unclean spirit goes out of a man, he goes through dry places, seeking rest and finds none.
So it is very clear that spirit is a soul of a human being or a living being.
Matthew 12:45
Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.
Jesus(pbuh) is refering his desciple as spirit.
Isaiah 11:2
King James Version (KJV)
2 And the spirit of the Lord shall rest upon him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and of the fear of the Lord;
Here Jesus(pbuh) mentioned as spirit.

For further understanding what is spirit we look from the wikipedia:
Spirit from wikipedia:
The English word spirit comes from the Latin spiritus, meaning "breath", but also "spirit, soul, courage, vigor", ultimately from a Proto-Indo-European *(s)peis. It is distinguished from Latin anima, "soul" (which nonetheless also derives from an Indo-European root meaning "to breathe"). In Greek, this distinction exists between pneuma (πνευμα), "breath, motile air, spirit," and psykhē (ψυχη), "soul" (even though the latter term, ψῡχή = psykhē/psūkhē, is also from an Indo-European root meaning "to breathe": *bhes-, zero grade *bhs- devoicing in proto-Greek to *phs-, resulting in historical-period Greek ps- in psūkhein, "to breathe", whence psūkhē, "spirit", "soul").

The word "spirit" came into Middle English via Old French. The distinction between soul and spirit also developed in the Abrahamic religions: Arabic nafs (نفس) opposite rúħ (روح); Hebrew neshama (נְשָׁמָה nəšâmâh) or nephesh (in Hebrew neshama comes from the root NŠM or "breath") opposite ruach (רוּחַ rûaħ). (Note, however, that in Semitic just as in Indo-European, this dichotomy has not always been as neat historically as it has come to be taken over a long period of development: Both נֶ֫פֶשׁ (root נפשׁ) and רוּחַ (root רוח), as well as cognate words in various Semitic languages, including Arabic, also preserve meanings involving misc. air phenomena: "breath", "wind", and even "odour".
Is it not now very clear that a spirit is nothing but a soul?
Three different word same meaning:

Nefesh == Instincts of the Soul
Ruach == Emotions of the Soul
Neshamah == Intellect of the Soul
Basically spirit is nothing but a soul for a living being rather than holy ghost.

REPLY:
//Does a spirit means holy ghost, not a human?\\
No Spirit Simply deosn't means Holy Spirit Nor Human soul

Matthew 12:45 - "Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there. And the final condition of that person is worse than the first. That is how it will be with this wicked generation.”

Yes spirit - is not simple spirit here, it is wicked spirits,

Like wise Spirit - has no effect with good or bad.
See SMELL, 
1) Fragrant SMELL; 
2) Rotten SMELL; 
I.e SMELL cannot be defined with out its character;

What is confusing here?

Let see .. .. ..

The claim Ruach is Just a spirit -> soul -> Human - - - Thus to Muhammad!
What a Absurd transformation!

Even if It is Human, No way that it should be Muhammad, It can be any Human Soul after Jesus. But unfortunately. Its Holy Spirit not a Simple Human Soul.

Let see from Above Points mentioned: (Pre-requisite, Have Example SMELL be in your mind)
Job 33:4 (KJV):The spirit of God hath made me, and the breath of the almighty hath given me life.
Reply : spirit of God, It is not merely spirit, it is SPIRIT OF GOD, we call this HOLY SPIRIT

Matthew 12:43 - When an unclean spirit goes out of a man, he goes through dry places, seeking rest and finds none.
//So it is very clear that spirit is a soul of a human being or a living being.\\
How it is very clear that spirit is Human soul???
Read the next versus - 
Matthew 12:44 - Then it says, ‘I will return to the house I left.’ When it arrives, it finds the house unoccupied, swept clean and put in order. 
Reply:
See the claim you had made "//that spirit is a soul of a human\\", If a soul left human,. there will be no more life in Him, he will be dead. But 12:44 Says "the house unoccupied, swept clean and put in order", The once wicked man, converted to Better Man, but not occupied by Holy spirit, even though it was swept and made clean. It lacked the Holy spirit to occupy the place , "the house unoccupied", Thus Wicked spirit enters and occupy his body, he became more worse.

Where is Human soul here?? How?? If Soul left Body , He should be dead, When Wicked came back no More Body should be there, else you are saying, RISEN FROM THE DEAD??   Unfortunately nope, He is not dead, See "it was swept and made clean".

So You can admit, Ruach is not Human soul RIGHT?? This is a GREAT example for Muslims Who claims, "Spirit is not always a Human soul"!

Matthew 12:45
Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.
Reply:
Here Continuation .. "Wicked Spirits" (Not simply spirit), enters Human soul, & made him more worse...
//Jesus(pbuh) is referring his disciple as spirit.\\ Here Jesus tells example for Jews came to test him, nothing said about disciples here! Not read or hear Bible, you must UNDERSTAND Bible.

Isaiah 11:2
King James Version (KJV)
2 And the spirit of the Lord shall rest upon him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and of the fear of the Lord;
Reply:
1) the spirit of wisdom
2) the spirit of understanding
3) the spirit of counsel and might
4) the spirit of knowledge and of the fear of the Lord
 From these , we can understand spirit is like smell, definition is its characteristics,  It can dissolve in another spirits. SPIRIT OF G-D, is called HOLY SPIRIT. All Others are called Spirit, good or bad depends on its specification.

Anti-christ spirit : 
1 John 2:22 - "Who is the liar but he who denies that Jesus is the Christ? This is the antichrist, he who denies the Father and the Son."
1 John 4:3 - "And every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already."

THUS MULSLIMS ARE HAVING ANTI-CHRIST SPIRIT;
That doesn't means Muslims are Having Anti-christ Soul, their Souls are defected with Anti-Christ spirit. (As said by Jesus in Wicked Man's example Matthew 12:43-46)


Thank you for your Question & chance for clearing!

Further reading Is perakletos (Comforter) can be Periklutos (A Praised one - Ahmad)?