I was having problems removing Silverlight 3 from the Add/Remove Programs feature. These are the steps I followed to get rid of it and upgrade to Silverlight 4.
Step 1 Create a new system restore point (if you feel like it)
Click Start, All Programs, Accessories, System Tools, and then click System Restore. The System Restore Wizard opens. Click Create a restore point, and then click Next. In Restore point description box, type a description. Note: The date and time are automatically added to your restore point. Therefore, you dont need them in your description. Click the Create button.
Step 2 Try Add/Remove Programs Again
In the control panel find Add/Remove Programs or Programs and Features whichever one you have. Uninstall Microsoft Silverlight if it is shown as installed by clicking on it and then clicking Uninstall or Remove.
If you still get an error message (for example a dialog appears saying that the msi file cannot be found) just continue on to the next step.
Step 3 Mess with things
Click on Start > Run Type in cmd and select OK. From the command prompt screen run the following commands
cd\ reg delete HKLM\Software\Microsoft\Silverlight /f
Close the command prompt window.
Rename the C:\Programs Files\Microsoft Silverlight folder to C:\Programs Files\MSSL
Goto Start >Run and type Regedit. It will open registry editor. Highlight My Computer. Click on the File menu and select Export. Make up a file name and save the file (on the desktop for example).
In the registry editor navigate to HKEY_CLASSES_ROOT\AgControl.AgControl Expand AgControl.AgControl
Select the CLSID key. Edit the (Default) value by right clicking and selecting modify and change it to {DFEAF541-F3E1-4c24-ACAC-99C30715084A} then click OK
Select the CurVer Key and change the (Default) value to AgControl.AgControl.1.0
Navigate to HKEY_CLASSES_ROOT\Installer\Products Find the key called D7314F9862C648A4DB8BE2A5B47BE100 and rename it to OLD_D7314F9862C648A4DB8BE2A5B47BE100
