Volevo installare SP1 di VS2008....
....ma mi sa che devo aspettare 
.
If you have installed the VS 2008 Tools for Silverlight 2 Beta1 package on your machine, you must uninstall it - as well as uninstall the KB949325 update for VS 2008 - before installing VS 2008 SP1 Beta (otherwise you will get a setup failure). You can find more details on the exact steps to follow here (note: you must uninstall two separate things). It is fine to have the Silverlight 2 runtime on your machine with .NET 3.5 SP1 - the component that needs to be uninstalled is the VS 2008 Tools for Silverlight 2 package. We will release an updated VS 2008 Tools for Silverlight package in a few weeks that works with the VS 2008 SP1 beta.
Fonte : Visual Studio 2008 and .NET Framework 3.5 Service Pack 1 Beta