Can't Logon To Windows Xp Home Edition

My problem is that every time I try to log in I get logged straight back out. Now i've looked at a lot of possible solutions like repairing it and checkign for spyware but they haven't seemed to fix the problem. It's possible for me to log in to safe mode but nothing else. The problems started after my friend networked his computer with mine temporarily so im guessing that the problem might be that its trying to log into his computer or something odd like that unfortunately I have no clue how to fix this. If anyone can give me a possible solution to this problem I would greatly appreciate it. By the way when it comes to these types of problems i'm pretty illiterate so if you could be as simple as possible that would be great.

re-install windows,start all over again...

I want to avoid that at all costs because I have massive files that I cant afford to lose and are too alrge to back up.