Microsoft .NET Framework 4.0 Beta 1 should be removed before upgrading to Windows 7
If you have been beta testing Microsoft .NET Framework 4.0 Beta 1, we recommend you install a fresh copy of Windows 7 on your machine. Alternatively, you may also choose to uninstall Visual Studio 2010 Beta 1 and .NET 4.0 Beta 1 prior to upgrading to Win7.
If you have installed .NET Framework 4.0 Beta 1 on your Windows Vista machine, after upgrading to Windows 7 you may encounter some problems. Managed code may not function, especially applications built to target .NET 4.0 Beta 1. Also, if you have any ClickOnce applications installed, Windows Explorer may crash continuously when you log in.
This issue has been fixed in .NET Framework 4.0 Beta 2.