I have a computer with a major problem; it gives a blue screen when the login screen loads. I've been using this guide to troubleshoot the issue, and now I've run into a problem.

I have determined the issue is not bad memory or a bad hard drive. According to the guide, this means the problem is in the OS. I've tried to follow the steps, but Windows (Vista SP1) somehow doesn't remember any changes. On every reboot, the computer is in exactly the same state it was in before. Any changes to system settings or files won't be recorded. As this means I can't check what is causing the problem, I can't fix my PC.

Is there a way to find out what's causing this? Is it just a mode Windows goes into to protect itself, or is it some other problem? Anything to help troubleshoot will be of great help here.

PS. I'm kind of new to this site. If I messed up, please tell me in the comments.