First time mine failed I found that the auto-updates was trying to download
and install SP3 at the same time as I had manually started it. I went in
and disabled the auto-update option.
After that it was time to kill my anti-virus/spyware programs as one of them
thought SP3 was malware trying to destroy the system...go figure. Never
figured out which of them was complaining but SP3 installed just fine after
they were disabled.
After the update and boot I had no further problems. Anti-virus/spyware
programs are behaving just fine and so is the rest of the system, so far...