So I was trying to install Visual Studio 2008 RTM tonight when I ran into a familiar but very annoying problem.  I had installed the Beta 2 of Orcas and used it for a while.  Now it was time to uninstall the beta version and get the RTM on.  The Beta did not uninstall very well so I had to follow the uninstall instructions from here:

http://www.microsoft.com/express/support/uninstall/

 Unfortunately that was not the only thing I needed to do.  After some careful use of Process Monitor I figured out what the installer was doing.  The setup was complaining that the Windows .NET SDK was still installed and it must be removed.  This product did not show up in the Add/Remove Programs applet so I dug in to the registry.

After some trial and error I exported the registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\7E715177542206733A97251FEA0FE924

I did this so that I could restore it if I were wrong.  Next, I deleted the key (7E715177542206733A97251FEA0FE924) and all it's subkeys (do this your own risk).  If you examine the InstallProperties sub key and look at the DisplayName value and you should see that it is "Windows SDK .Net Tools". 

This worked for me and ended a very frustrating night.  I hope this helps you as well!