according to microsoft support site, there could be few reasons
1) driver issue , you might need to press F6 and use the mobo sata driver floppy (but if you didnt do that in previous times you reinstalled or alredy did it than thats probably not the issue)
2) filesystem curruption
3) boot virus
lets assume its 2/3 , load the recovery console read here how
http://support.microsoft.com/default...b;en-us;307654 and do chkdsk , if that wont help then youll have to repartition that hard drive (means youll have to delete all of its partitions and recreate,but if you have only 1 hard drive then youll likely lose alot of important data so if you need to backup some data before doing so use
ubcd4win or
reatogo and burn the data to cd/dvd media)