why don;t you try to access windows throught safe mode, once you are there create another accound, and scan your computer for viruses. Likewise, do;t set any password for your accounts so windows can access your accoiunt without having to show you the login screen, if that works then i guess there is a glitch or a virus doing that.

you might want to use your xp cd, and access command line from it, then restore your computer by going to the restore folder and using the restore program that is in that folder. (you can do that from safe mode too, if you have access to safe mode then do it from there.

btw, if you get your computer back in business, try to use software such as ghost to create a backup of your computer in case you run in trouble in the future, that will free you from a lot of trouble. I know ghost might be a little bit expensive, but there are always a way to get stuff for free