Computer Problems

My computer keeps freezing up for no reason every time I log in. This happens on both of the hard drives, one being Ubuntu Linux and the other being Windows XP. I've been dual booting for awhile now, and never had a problem with that. I guess it's been freezing up here and there for a long time, but now it's freezing up soon after I log in, or even if the log in screen sits for a few minutes. It's dieing fast and I don't know why. Of course, I need to back up and have no where to go with my stuff. Also don't know how far I'll get until freezing up. Guess I'll just have to get what I can. It's so unstable that I won't boot up until I have a back up plan in place, just in case it never comes on again.