Out of interest are any of you running Zonealarm.

I had a similar issue a while back and it magically went away when Zonealarm prompted to allow Avast emergency update web access.

I was able to replicate the issue several times using a ghost image and found the best was to resolve was to disable ZA from starting at boot, leave the PC on for an hour.

Then start Zonealarm and re-enable start on boot, wait a while for access request and allow.

After following the above procedure reboots/shutdowns worked fine again.

I run several machines on XP pro but only one was ever affected by this.