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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.