"Need to uninstall older version", Easiest workaround
I spent far too long trying to delete all traces of my previous install to appease the installer and was unsuccessful. So here's how to just get to the program(which is otherwise wonderful) nice and easy..
1: Go here: http://www.modthesims.info/download.php?t=409950
2: Download Sims2PackInstaller_NoInstall.zip and extract it
3: Go to a pack file and select "Open with", browse to the folder you extracted the zip file to and select "Sims2Pack Clean Installer.exe" (be sure to check "Always use the selected program to open this kind of file"
And it should be good to go, hopefully I saved someone a few minutes.