I think he means he had an arred version of the game and then he got a legit one.
If so, then your problem is that probably the uninstaller was just borked, that happens even on legit versions of the game, and thus leaving trash behind any uninstall or worse, failing to succesfully uninstalling the whole thing.
In order to fix this, you'll have to remove all files manually and then run the RegEdit (don't know how to do that on Vista
), once there, do a search of "Eletronic Arts" or "Sims3"/"The Sims 3" and remove the files that are shown on the results, but BEWARE!, make sure you don't delete other non-sims3 values, otherwise you will royally f*ck up your computer.
After that your new install of the retail version of TS3 will run normally, or at least that worked for me, don't do anything I wrote above if you're not 100% sure about it, I don't want you to come here crying if you suck at Registry Editor and did it all wrong.
Hope that helps, if that's the case.