Sony Media Manager 2.3 - wrong privileges

I'm trying to install Media Manager 2.3 to go with Sony Vegas 8, but when it reaches the actual install process it instantly pops up a message saying:

Please make sure you have local system administrator privileges on this system. Cancelling the installation.

And does so. I've tried starting Vista in safe mode and installing, and I know that I have admin privileges. Any ideas what might be stopping it from running?

Thanks,
Phil