Make sure that ALL the files for The Sims were lost.
First go to
C:/Program Files/Maxis/The Sims
and see if there are any files there. If there is, then look for a file named "Sims" It will be an EXE or an Application file. If this file is there, double click on it. If The Sims loads, then you didn't lose the folders and files.
If The Sims is still there, then just procede with the installation of the new expansion pack.
If The Sims isn't there, then you may have to download a program called Cleanser. This program was created for The Sims to erase the registry files so the game could be reinstalled.
You can find it here: