Black screen window

Hello,
Last night ,i am transfering files from mobile to my pc, suddenly it show virus message that virus is detected and removed and i pressed Ok and countinue my work, it again show the same.When i restart my computer a black screen appears with a cursor, and no logon window has displayed, i try to run it in a safe mode , but it also won't work there. I have window XP, and my nortron antivirus is uptodate, Tell me what the solution of this,and how can i take the backup of my data