Disable the automatic updates & bits services, delete the entire softwaredistribution folder, re-enable services. Usually resets the system to check again for new updates.
Also, depending on where you are with updates, XP can take extraordinary times seeking out, downloading, and installing updates. The algorithms to check for updates, I believe it grew exponentially the more updates that get released. If you do a fresh install with an SP3 cd, the initial update process afterwards easily runs 4 or 5 hours.