Explanation:

Earlier I had Visual Studio 2008 Express edition installed on my home PC. I installed the ASP.NET 3.5 Extensions CTP there too.
Now I installed the Professional Edition of the Visual Studio. Suddenly I found that I was not able to see the MVC Framework templates. So I thought I should reinstall them.

When I tried to Uninstall I found that it was looking for a file C:\windows\installer\aspnetext.msi. This actually did not exist.
After that I did a manual solution which might also make your life easy 

Please follow these steps if you are having the same problem:

 
1. In the Folders Properties of your file Explorer set "show hidden files"
2. Unzip the downloaded package "ASPNEText.Exe" of "ASP.NET 3.5 Extensions" with any Zip Application (I used WinRAR)
3. You will find the above required file "aspnetext.msi" under folder named "aspnetext".
4. Copy and past it to the desired location (you wont be able to find the location if you did not performed step 1).
5. Now you will be able to reinstall/uninstall the Extension