System Hangs On Startup

I have an old computer. My uncle threw it together from scraps and sent it to me. It ran very well for about a year or two. It runs off Windows XP Professional. The problem with it now, and i just cannot figure it out, is when i go to start the computer. The computer boots normally, and so does windows, however when it displays "windows XP" just before showing the users list, it freezes. The mouse still is able to move, but it is as if there is no more users at all on the computer. I have been tinkering with it for over a year and still cannot find a way of getting into it, or getting it to work. Reformatting the computer is not an option because i have important information on the hard disk. Got any advice?