I know a bit about Windows operating systems and have successfully fixed problems such as this. Surly there are other ways, but in times of need taking "the time to learn new trix" can be replaced with "getting it done and over with"
1. Do a virus scan. ( For the hell of it )
2. Back up important files not anything that will infect any machine (*cough*PS GAURD*cough*) DVDs or on an external HD or CD's a DVtape or anything that will contain your data so long as it's off your HD.
3. do an F Disk.
4. Re-install Windows.
Install and update your Antivirus, then Upload your backed up data into your formatted HD, Virus scan it with glory. Done.
If this is the case
If your running Windows XP and, If your PC was not originally a Windows XP machine, and you want to re-install windows XP. (I would have to hit the books on how to do that again cause XP does not understand DOS). So install "Windows NT or 98 OEM" And when it is complete, install Windows XP as an upgrade.
Stuff you probably don't care about
Somewhere along the lines you may be given the option to set your HDD as NTSF or Fat32 So look out for that.
NTSF has a better compression ratio from what i have read about it, and offers a better performance for your operating system, NTSF takes note on how files fluxuate in size slightly while in use, thus allowing the HDD to fragment slower. If it's an upgrade from Windows 98 you'll most likely be stuck with a FAT or FAT32. Honestly if it's NTSF or FAT32, i can't tell the difference when it comes to the performance really, After an F Disk i'd be nice to your PC and defrag it often enough. I read how NTSF is most effective for larger HDD in the 128 GB range, As it was described to me, It only offeres 4 GBs of file fluxuation space. I think that is alot considering your not running files that large.
A note on Maintanance not for everyone
-Stay away from porn sites and treat your PC like a lady.
-Buy a cheap peice of shyte thing of the past for seperate use, and treat it like a whore.