windows XP update and installshield hang after upgrade

I just upgraded from XP home to XP pro, more or less
successfully. But when I try to run Windows Update my
program hangs after all updates are downloaded, but
before any are installed (I tried waiting all night, but
the bars on the "install" progress never lit up beyond
zero).
I also found that when I tried to apply a downloaded
upgrade to Dreamweaver (6.1 to 6.0) the Installshield
Wizard program froze at the same point, with 0%
progress. Maybe the two problems are related.

Any ideas, please?

Thanks

Gareth