Am I right that you wrote your question from some other computer? Obviously, if you wrote it from your own computer and were able to, then there would be no problem, right?
Do you have a virus scanning program for your computer? I would absolutely do a full system scan. Could very well be a virus or worm or whatever.
It's unusual that something happens so suddenly and there's no reason for it. Do you have a reboot disk or floppy (I HOPE so). You could put that in to restart your computer so that you could at least reboot it and start a virus scan on it.
Have you tried to start it in safe mode? Or, does it do nothing at all and you can't even get that screen up?
I once had a virus or something that wiped everything out but it wasn't like what you're saying. My screen went blue, all blue. And, my hard drive was totally wiped out. I had to call Microsoft to help me set my computer up again. There was NO Geek Squad at that time. On an off, I was on the phone for like 7 hours with their reps (Microsoft/Windows) but they did get me up and running again and restored my system by telling me what to do.
You didn't say how old you computer is but if you had XP on it, doesn't sound too old. I'd not chuck it like people are telling you to do. Duh! Like that's not expensive to do!?!
If all else fails, try using a Geek Squad but, I would first email or contact Microsoft to see if they can help. If there's a charge, it will be cheaper than a new computer.
OR, do you have the XP software? Maybe you can just reinstall it???? I have Vista and there's a program where you can either delete, repair or replace your programs. Can you "repair" XP somewhere, using your software?
Gosh. That's a bummer. Hope you can figure something out or that something I said can help.