Trojans on winlogon.exe

Hello there

Sorry to bother but last christmas my PC was hit with a few trojans and after multiple scans the culprits were found on the C:\WINDOWS\system32\winlogon.exe file. As a result they've b*ggered up the logging on procedure so that any user trying to log on is immediately logged out.

I used avast! antivirus to remove some of the additional malware on the PC but every time I tried to scan the system32\winlogon.exe file individually the window would disappear and the scan would discontinue.

Another strange happening about it was that the PC was infected roughly around the 13th dec and it only allowed the user to log in only once (albeit with no taskbars/icons) but a week later the PC refused to let anybody log in.

I'm running on Windows XP incase that helps and unfortunately the PC won't allow me to log on in Safe Mode (unless I'm not doing it properly; and I know it's F8) and I'm also in the vein of trying to keep my documents/photos intact so any advice on retaining them would be suitable as well as finding a backdoor way of logging onto the PC and removing the miscreants.
Thanks.