Windows XP silliness

Another strange problem today while trying to install Firefox 3 RC3 on a Windows XP SP2 on a corporate laptop. Almost immediately after I run the installer:

"Sorry, Firefox can’t be installed. This version of Firefox requires Microsoft Windows 2000 or newer.”

WTF? This is a Windows XP SP2, you moron !

I remember having the exact error with one of the Beta releases. A reboot does not make a difference, I have administrator privileges, so it must be the braindead anti-virus, right ?