Hi, I used to have similar problems on my old PC. 4 possible areas to look at.
Whatever it might be hope youve first backed up all your data, emails etc. before attempting any of the following.
1. Check your bios setup in general to conform with your other cards, especially your graphics, setup conflict ?
2. Either a faulty power on switch or one or more of the motherboard mounting screws to the chassis is leaking ground intermittenly. (for me, this was my actual problem)
3. I had alot of gunk stuck in the cooling fans over the CPU, graphic card and all over my mother board, might have caused excessive heat build up. Theres free software available to see running temperature.
4. Bad sector/s. If you havent already checked, Run chkdsk /? at command for required switches. However Im uncertain if chkdsk looks at the (MBR) master boot record in the reserved hidden area of HD, which could also go bad though very rare. If you do find bad sectors, you'll need a low level format to write out these sectors and reinstall Windows.
chkdsk sample taken from MS site:chkdsk (drive)<space> /p<space> /r
Where /p Does an exhaustive check of the drive and corrects any errors.
/r Locates bad sectors and recovers readable information.
Note If you specify the /r option, the /p option is implied. When you specify the chkdsk command without arguments, the command checks the current drive with no options in effect.
Do bear in mind, dealing with MBR is pretty advanced stuff and its a last option.
You got to determine if you do actually have a corrupted MBR, see google how :}
More reading on MBR, you might want to look here-
http://techreport.com/forums/viewtopic.php?p=1147088 OR
http://www.ntfs.com/mbr-damaged.htm OR
http://helpdeskgeek.com/how-to/fix-mbr-xp-vista/ OR
http://www.ehow.com/how_5087860_fix-mbr-windows-xp.html OR
http://support.microsoft.com/kb/314058Its all I can think of for now. Good luck, mate.