Windows XP: failure to log on in startup

[Dark Lord of the Bith]

So, my lappy's been acting fishy lately. It started around when my roommate "helped" fix a previous problem, which was eventually resolved, but now there's this new problem. Moral of the story: never trust a computer science major.

Whenever I start up my computer and log on to Windows, my computer never completes loading Windows Explorer (the Start menu never appears, etc.), though it gets to the point of loading my wallpaper. My current workaround is to ctrl+alt+del and Shut down -> log off user, tell it to End Now on Windows Explorer, wait a minute or so, then try logging on again, which is always quick and successful. It basically results in my startup being about 3 minutes long, which is incredibly annoying, but not catostrophic.

It only happens when the computer powers on. Once it's working, I can log off / log on at will.

I'm guessing I'll need to fix something in the registry, but I have very little idea what to do there. Can anyone help?

[Dark Lord of the Bith]

Yes. Also, there's a seemingly random chance that it works the first time it boots up, though that case is less likely.