READ: Installation issues with Windows Phone tools and Silverlight 4

Question

Problem: The installation of Windows Phone Developer Tools (WPDT) will fail if you have previously installed the latest update of Silverlight 4, which was published to Microsoft Update on Thursday 6/3, versioned 4.0.50524.0. This failure occurs when WPDT setup attempts to install Silverlight 4. 

WPDT Versions Impacted: April CTP and April CTP refresh

How to work around the issue:
If you run into this error while trying to install latest WPDT , you can solve it by doing the following:

1. Go to the Programs and Feature control panel. 
2. Locate the item named Microsoft Silverlight and uninstall it.
3. Re-run WPDT setup.

This issue will be addressed in a future release of the Windows Phone Developer Tools.