On a couple of occasions I have attempted to upgrade a PC from SP2 to SP3 and after the installation, the PC blue screened on start up - no safe mode or anything. Worked with MS tech support and they had no suggestions other than to uninstall Symantec Endpoint Protection, then connect to the Internet and run live update. He didn't have an answer when I asked if he would put a PC without virus protection on the Internet for a number of hours to download and install this large service pack.

If MS is aware of issues (with virus checkers or whatever) after all these years that SP3 has been out, they should run a system check before it installs SP3 to alert you so you don't hose your PC. Had to reimage the box (luckily there was no data on it).

So I've learned not to try to upgrade to SP3 on important PCs. And if I later have to try to install SP3, I'll image the box first, then manually download SP3, uninstall my virus checker, and install SP3 manually (not via live update), then reinstall my virus checker - if SP3 installs successfully.