Computer Freezes at logon screen.

Just recently on one of my XP machines, once it gets to the logon screen, if i touch any key on my keyboard, my computer will in a way freeze up. I say this becauase i can move the mouse before i touch the keyboard, but once i touch a button on the keyboard i cant move the mouse or type anything, although if i press the power button on the tower, the box that says “windows is now shutting down” will pop up, so its not TOTALLY freezing up, i just cant move the mouse or type. Any ideas?