Vista and Win 7 have done that to me, it is frustrating because, at least for me, these kind of problems come and go and I can't reproduce when need it.
Sorry, not really an answer for you, just a little more salt to the wound I guess
All I can say is try to set a restore point, go back to a couple or few days to see if that solves your problem.
Perhaps, restarting your PC with the CD in the drive and using Repair might help getting everything back? Now, being said this, be careful because you may end up making a completely new install and loose your data in the process or make a new windows folder (kinda like a dual booting) and still have the problem in the OS you now use.
Again, sorry, it seems I am giving you more frustrations than answers, hopefully, someone with more skills will chime in soon.