i would recomend taking your hard drive out and putting it in someone elses computer (if you can) and use their antivirus software to scan your system
go to searh
enter no criteria
hit enter
when its done finding all files sort by size then date
size should tell you whats taking up alot
date could indicate if a virus has replicated itself a bunch
other problems could be a bad partition table
bad sectors on drive
memory holes in program
drive geometry off
etc
without more info its hard to say
but i allways keep and empty usb2 housing handy so i can slap a hard drive or cdrom or whatever in it quick
alows me to scan other peoples drives without cracking the case open
in your case it would allow you to take your hard drive out and hook it up to someone elses computer with minimal hassle
such housings can be gotten relativly cheap
10 - 40 dollars (ball park figures)
mine was 23
also its good to have recovery enviroments handy
such as knoppix ( a version of linux that boots off cdrom not hard drive)
or windows pe
http://www.nu2.nu/pebuilder/
this allows you to make a bootable version of xp that runs of cdrom not hard drive and can be used to fix problems etc...
