Two Different Versions.......

[ash2dust2]

You probably could do it under one OS, but that would be alot of work.

Get the game running at the old version you want, save all version/folder relevant registry entries.

Install, get the game running in another folder at 1.5, save all version/folder relevant registry entries.

Import the reg for what version you want.

Gamespy, all seeing eye, and any other external game launchers will only point to one directory or another.