Go to Server Manager -> Server Roles -> Add Role -> Select Web Server (IIS 7.0) there. This will install Web server.
Now go to Server Manager -> Roles -> WEb Server -> Add Role Features -> Select IIS 6.0 Compatibilty option and other IIS 6.0 related options from the list.
The figure below shows Server Explorer on Windows Server 2008 with IIS 6.0 enabled as part of Role Services.
Once you are done with the changes, Save them and your SQL Server will now be able to find IIS 6.0 installed on the system. Run the TFS 2008 set up and see if it passes the system compatibility check.
Hope this will help you in case you find installation issues with TFS 2008 on WS 2008, SQL Server 2005, Windows Sharepoint Server 3.0.
bye for now.
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