and if that doesnt work....... boot off the xp disc and do a repair install, making sure that you dont format the drive.
that should fix any issues with it. I would also make sure that all the cables in your computer are plugged in, esp the harddrive ones.
how old is this computer? the harddrive especially. you might have a harddrive that is failing
has the harddrive gotten any noisier? also when is the last time you cleaned dust out of the inside of your pc?
sorry about all the questions. just at this point , with the info you have given, there is so many things that can possibly cause this. time to start just trying things and narrow down the list