Check your "Add or Remove Programs" (XP) or "Uninstall a Program" (Vista) control panel option for:
Microsoft .Net Framework 1.1
and
Microsoft .Net Framework 1.1 Hotfix (KB####)
(Game needs both 1.1 and the 1.1 "service pack").
If they're both listed try uninstalling them starting with 1.1 and then the "hotfix" and downloading and reinstalling them from the links below (if only one is listed uninstall just that one and then reinstall both):
Microsoft .Net Framework 1.1
The 1.1 Service Pack (Hotfix)
If it keeps up though definitely follow up with a
support request if you haven't yet.