Is windows unable to find c:/Program or c:/Program Files?
I don't have the slightest idea what that problem is, but since it just happened randomly and not as a result of you messing with your boot settings or system files (I hope) then it does sound like a corrupt file or registry. This will tell you for sure:
Download
HiJack This (a registry scanner) to a floppy disk. Restart in safemode and run the scan, save the logfile to the floppy. On another computer or the same one if you can still access the internet, go to this website:
HiJackThis Log File Analyzer. Upload or copy/paste your logfile and see what comes up.. if there are some huge red "OMG THIS THING IS A VIRUS! IT IS EATING YOUR SYSTEM AND KEYLOGGING YOUR EVERY MOVE" then you'll know what the problem is.
If there are some inconclusive results, don't panic, this log will help other experts troubleshoot and is very useful should you have to take your problem to a better tech board...which incedentally is what I'd suggest you do anyway.