Hi Carol:

I was running Workstation installations at a client last week (admittedly different from a service pack). On all computers, it would stop at the same percentage and go no further. There was no message and the installer wasn't really frozen (i.e. I could click on Cancel - though it didn't actually cancel). The solution, after much gnashing of teeth, was to temporarily stop the anti-virus software. I realize from your question that you've already tried that, but the symptoms sound very similar.