The setup for VS Orcas and the .NET Framework 3.5 has a list of pre-release installations that must be removed before installing. When you uninstall beta1 or a CTP, it should removed the components that would block; however in some cases you had more than one product installed or there was some issue or problem on uninstall.
You might see an error message like this -- "These pre-release products might include, but are not limited to, the following: - MDSN Library for Visual Studio Orcas - ENU".
You can look in Add/Remove programs or Programs and Features on Vista and above to remove any of the Orcas pre-release software (most likely beta1).
If you don’t see any, you can use this tool that Aaron Stebner has posted called msiinv.exe which will enumerate your installed programs.
If it still doesn’t install after removing all of the VS Orcas or VS 2008 bits from the output of msiinv.exe, you might have a different issue.