When I ran WU the service pack seemed to install smoothly. But when it rebooted it froze about 60 seconds after logon and did so for every subsequent reboot. Also on booting Windows Defender kept reporting that "mswsock.dll was being changed by a known application". Spent a whole day researching this and trying various fixes - nothing worked - finally gave up and used system restore to go back to a non Sp1 installation. PC working fine - looks like I will have to give SP1 a miss - maybe Microsoft will get it right with SP2. Mercifully this was just my home PC - we still use XP at work thank god!. If this happened on all the PCs at work it would have been a nightmare.