Probably incompatibility with windows server 2008
I've run into this problem as well.
1. Already downloaded the full installer
2. Tried install, the license number is not pre-entered
3. Read this forum, and since it mentioned Internet Explorer, I tried disabling IE ESC (Enhanced Security Configuration) for Administrators. Did not work.
4. With ESC off, I tried running installer as an Administrator. Still did not work.
It's a free product, so clearly we have no grounds for complaining. But it would be nice if the minority of people who run Windows Server 2003/2008 for testing (as opposed to running an enterprise server environment) could also benefit from AVG's generosity.