Sorry, but that has nothing to do with the UC installer. I tested that myself before releasing it.
That's fine, maybe I should've prefaced my bug post with "on my machine...". It does it consistently here. Every time I run the installer it shows the last install path I used.
I've installed UC to a different path/uninstalled and tried again three times now. Each time the installer shows me the last path I entered. It's never shown me "$PROGRAMFILES\3000AD\Universal Combat", which on this machine would resolve to "C:\program files\3000AD\Universal Combat"
While the snipet of NSIS code you posted clearly shows the InstallDir variable being set to what you want there's not enough there to verify if the variable is being clobbered later on or if the Install Path edit text box is being populated from an odd Registry entry as opposed to the InstallDir variable.
Just pointing out something anomalous.