Both Boot camp and Parallels had been running well for ages. Upgraded Parallels to 5600 and boot camp to 2.1 in preparation for installing XP SP3. All still fine. Then installed Windows XP SP3 in Boot Camp with no problems. When I tried to start Parallels I got the dreaded message "autochk program not found - skipping autocheck". Parallels was stuck in an endless loop, restarting after a few seconds and getting this same message again. Paused parallels and did some internet searching. Discovered a thread in this forum which suggested going to the Advanced option in the Parallels drive configuration screen and removing all Parallels information from the Boot Camp partition. That could not be done with Parallels running so I ignored the dire warnings and forced the Parallels virtual machine to stop. Removed the Parallels info and YES it started up just fine, thinking that this was the first run of a new Parallels installation. Didn't even have to reinstall Parallels Tools in Windows. BUT I got a Microsoft message that because of a significant hardware change my copy of Windows needed to be activated, followed by nasty warnings from the Windows Genuine Advantage anti-piracy software. The good news is that when I chose online activation, it went smoothly taking only a few seconds, with no need to phone. If anyone else is about to upgrade Boot Camp to XP SP3 while running Boot Camp, I suggest removing the Parallels information BEFORE you first start the upgraded system in Parallels.