Hi Aces,
if your OS is WindowsXP it would be helpful to use my little tool for cleaning the registry of SH3 entries:
http://www.subsim.com/radioroom/show...70&postcount=5
Since the registry entries don't affect the game itself in any way [they're only InstallShield related] you're able now to install another SH3 [into a different folder] without uninstallation of the old one.
Cheers,
SquareSteelBar