If you can change the directory to which it installs, try doing that. Dunno why it would try to upgrade, but usually if there is an upgrade option there is also a 'create secondary install' or something like that for an option.
Apparently someone else had had this problem, and he troubleshot it on
his site. Sounds like it would be some unique things that you would have installed, so I'm not sure if the top part would help much. Read further down and it should show you where the registry keys are that may need to be removed.