My computer won't let me login....... [off-topic i know... but need help...]

My mom told me that she said it's a virus when she trys to go thru the login loop... she download norton 360 then she went to work... i scan the computer, and it found 1 virus called win 32 trojan something like that and i tried to remove it and it unabled to clean it off....then my computer froze so i restarted it, the welcome screen came back and it show the desktop and went back to login screen, i logged in and it logged me off itself, i tried logging in numerous times, it won't let me in. i tried going in safe mode and safe mode with networking, it did the same thing as i try to log in....it never let me in at alll....
i don't want to format my computer again because i want to keep the stuff i wanted. i formatted three time. the last time is last weekend, the other two time is long time ago..... sooo please can someone help me with this.... it's window xp. i'm using my brother's computer.. and i don't have a cd for repair... so what can i do??