This could be a power supply problem, particularly if you have an 8800 that
requires its own power connector. That would be my first 5 guesses.
If your computer is a year old and you are running with the power supply
that came with the case it is about time for a melt-down.
If you end up getting a new power supply get a name brand, e.g. Cooler
Master or Antec or OCZ (there are others) that is at least 600 watts. For
upper end NVidia cards 650 watts is a good minimum and some recent ATI cards
need even more watts.
This sounds like a hard ware problem. It is not likely your motherboard,
videocard or hard drive. You can see if you can run the Microsoft memory
tester as this usually rapidly identifies bad ram.
Before considering a Windows reinstall that will trash your hard drive, and
from what you write sounds pointless, see if you can plug your hard drive
into another computer and off load whatever files you need.