Vista - Why does it say I need admin priveleges when I know I have them?

I am trying to install Sony Vegas pro 9.0 on my computer but I can't even open the trial version. Every time I do I get a message saying "Registration requires administrator privileges. Please run Vegas Pro 9.0 from an administrator account to complete registration".

I've confirmed and reconfirmed that my user account has administrator privileges. Here's what I've tried.

1. Right click - "Run as administrator." Failed.

2. Right click - properties - compatibility - check the "Run as administrator" box. Failed.

3. Activated Vista's hidden administrator account, made sure it really did have admin privileges, installed and ran it from there (also granted guest account admin privs and ran from that account). Failed.

4. Disabled UAC. - Failed (re-enabled afterwards)

5. Ran it from command prompt as administrator. Failed.

I've also tried numerous combinations of the above and every time I try to run the trial I get the same message. Also, I re-installed the trial every time I tried something different so there would be no mistake. How can I convince it that I do indeed have administrator privileges?