Wow, sounds like its going from bad to worse
I would be worried about my drives being corrupted if this was my setup and things were deteriorating like that.
My suggestion, though a little extreme, take out your drives and plug them into another PC if you can to back up your data/files.
After that start a rebuild of the PC with the base components in and a clean install of windows (7 or vista). Then add your other components one at a time checking they are fully working before switching off and adding the next.
If you want to try and check your PC more before going to that extreme how about downloading and burning a copy of Ubuntu or other Linux that boots from CD? At least you could take a look at your drives and USB ports knowing that its not windows that is causing errors.