Seems like a very weird issue.
Do the games show as installed in the launcher ? Meaning do the icons of SN and ST have a version number ?
I'm using Windows Vista, which hasn't ever presented any problems installing/running the games until now.
I bought both of the games in DVD hardcopy from Walmart, and they both show up as installed on the launcher. When I run the game with them both installed though, The supernatural content and all my other expansion content shows up, except for Showtime.
Another thing I forgot to mention is that since I installed Showtime after Supernatural, when I tried to launch the game with the showtime disk, the launcher said that it needed the supernatural disk in order to run.
I just wished I had a solution to all these weird installation issues.
You always need the DVD of the latest EP EA released, so that's normal, it does mean that the game is recognizing SN ok, just makes me wonder what causes the stuff not to show up.
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Try renaming the folder documents\electronicarts\thesims3 in something like documents\electronicarts\thesims3_backup then start the game to have the game create a fresh user data folder. Apart from uninstalling everything, cleaning registry and reinstalling, I can't think of anything else to try.
