I had Visual Studio 2008 Team RC installed on my laptop and I wanted to install the RTM version so I uninstalled the RC and launched the RTM setup but the setup would fail.  After a little digging and reading ScottGu's blog, I figured out that the Visual Studio setup install numerous products by launching separate installations.  So when you actually uninstall VS, you're not uninstalling everything that was actually installed.

http://weblogs.asp.net/scottgu/archive/2007/11/19/visual-studio-2008-and-net-3-5-released.aspx

People often ask me for suggestions on how best to upgrade from previous betas of Visual Studio 2008.  In general I'd recommend uninstalling the Beta2 bits explicitly.  As part of this you should uninstall Visual Studio 2008 Beta2, .NET Framework Beta2, as well as the Visual Studio Web Authoring Component (these are all separate installs and need to be uninstalled separately).  I then usually recommend rebooting the machine after uninstalling just to make sure everything is clean before you kick off the new install.  You can then install the final release of VS 2008 and .NET 3.5 on the machine.

Looking at the Programs and Features entries, I uninstalled these products in this specific order, then went to install VS 2008 RTM without a glitch.

Microsoft Visual Studio Team System 2008 Team Suite
MSDN Library for Visual Studio 2008 - ENU
Microsoft Document Explorer 2008
Microsoft Visual Studio Web Authoring Component
Microsoft Windows SDK for Visual Studio 2008 .NET Framework Tools
Microsoft Windows SDK for Visual Studio 2008 SDK Reference Assemblies and IntelliSense
Microsoft Windows SDK for Visual Studio 2008 Tools
Microsoft SQL Server Compact 3.5 Design Tools ENU
Microsoft SQL Server Compact 3.5 ENU
Microsoft Device Emulator version 3.0 - ENU
Microsoft .NET Compact Framework 3.5
Microsoft .NET Framework 3.5