I emailed Opus to find out why the Cubase SX update won't run after the XP tweaks. Here's the response:

"...here's what I found. Don't put the dongle on until you are done installing all the software and updates! Have you tried that one? Seemed to have worked for me. I think it's due to the portable media serial Service thang. not sure entirely but again, installing without the dongle attached seemed to have worked for me!
Hope that helps!
Cheers
Gary"

Can't try it now because I'm stuck here in my cubicle doing helpdesk till later.

Thanks Gary. I'll give it a try and let you know if it works.