I can't log in window xp....Please help!!!?

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I ran AVG virus scan and found trojan horse and some other viruses on my computer. I quarantined them and had to restart. I tried logging in window using a correct user password and it kept saying "saving..." then log off automatically. So I tried to start in safe mode using F8. I tried all options ( safe mode, safe mode with command prompt, previous good configuration, etc.) and it still logged off by itself. I can't even type a command. After I clicked on command prompt, it went straight into window log in and the same thing happened again. I don't know what else to do except reformat. But I don't want to because I have a lot of important files that I haven't saved. Can anyone help me? I need step by step instruction. Many thanks!