You may need to connect to the system registry using another computer.
When on a network, you need to open a remote registry off the working PC, and connect to the registry of the computer you can't logon to. Sometimes this is caused by a virus/malware, or a remote control program such as PC Anywhere.
This is a good link to a fix for this with some details. If you have issues following what they want you do to, you need to locate a computer savvy friend or a computer shop that can do this.http://en.kioskea.net/forum/affich-22087-xp-auto-log-off