To complement a previous post for Visual Studio 2003 and 2005, this registry key will add a context menu item to windows explorer.  The menu item opens a command prompt with the vsvars32.bat from Visual Studio 2008.

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\Directory\shell\Visual Studio .NET 2008 Command Prompt]

[HKEY_CLASSES_ROOT\Directory\shell\Visual Studio .NET 2008 Command Prompt\command]
@="cmd.exe %1 /K \"C:\\Program Files\\Microsoft Visual Studio 9.0\\Common7\\Tools\\vsvars32.bat\""