If you have experienced some problems with the installation of the Windows Phone Developer Tools, you may want to check this solution provided by Michael Klucher.
If you’ve run into some trouble installing the latest version of the Windows Phone Developer Tools from http://developer.windowsphone.com we’re here to help!
Specifically one issue with installing the Windows Phone Developer Tools with Silverlight already installed on your machine is coming up more and more, luckily Aaron Stebner has provided a great post on working around the issue.