With the release of Silverlight 3, you may run into issue where silverlight 2 application may not work as expected. Please read this article on msdn Ensuring That Your Silverlight 2 Applications Work with Silverlight 3
You may need to revert to Silverlight 2 runtime to make sure that the issue has to do with the Silverlight 3 runtime. First of all in order for you to revert back and forth between Silverlight 2 and Silverlight 3 runtime, you should not uninstall Silverlight 2 runtime from your machine. Silverlight runtime gets loaded based on Silverlight plug in which is a COM dll(npctrl.dll).
COM Dll CLSID is “DFEAF541-F3E1-4c24-ACAC-99C30715084A” which remains same in silverlight 3 basically, you can’t run Silverlight 2 and Silverlight 3 runtime(CLR for Silverlight) side by side. Once you install Silverlight 3 runtime, browser running silverlight 2 app will load the Silverlight 3 runtime, you can verify that by looking at the loaded silverlight runtime dll in browser process space. By Default, Silverlight 3 will be installed in “C:\Program Files\Microsoft Silverlight\3.xxxx” so in order for you to revert back and forth all you need to do is register and unregister com dll.
You can simply create a batch file to do the job
regsvr32 /u /s “C:\Program Files\Microsoft Silverlight\3.0.40624.0\npctrl.dll”
regsvr32 /s “C:\Program Files\Microsoft Silverlight\2.0.40115.0\npctrl.dll”
Revert to silverlight 3 runtime, click to download
regsvr32 /u /s “C:\Program Files\Microsoft Silverlight\2.0.40115.0\npctrl.dll”
regsvr32 /s “C:\Program Files\Microsoft Silverlight\3.0.40624.0\npctrl.dll”