Please note that as of October 24, 2014, the Nokia Developer Wiki will no longer be accepting user contributions, including new entries, edits and comments, as we begin transitioning to our new home, in the Windows Phone Development Wiki. We plan to move over the majority of the existing entries. Thanks for all your past and future contributions.
How to reinstall 3rd Edition SDK
If you reinstall 3rd Edtion SDK, you'll find _2 end of the dir like C:\Symbian\9.1\S60_3rd_MR_2\, even though you have uninstalled the old sdk's and also deleted the S60_3rd_MR directory.
In this case, you should remove the old S60_3rd_MR directory, and deleting the HKEY_LOCAL_MACHINE\SOFTWARE\Nokia\com.nokia.s60 key will revert the "counter" back to zero, and give you an installation with the correct SDK name.
To reinstall 3rd Edtion SDK:
Step1: Uninstall old 3rd Edition SDK.
Step2: Run cmd Regedit.
Step3: Delete the HKEY_LOCAL_MACHINE\SOFTWARE\Nokia\com.nokia.s60 key.
Step4: Install new 3rd Edition SDK.
If you dont want to play with Regedit
After uninstalling the SDK there are still some personal files in the X:\Symbian\.. directory which do not get deleted. Take a backup of all these in some other drive or rename this root directory. Now install the SDK all over again.