not sure. When I played it I believe i picked the option that installs both discs on my computer (so you don't have to swich between discs). Are you really short on computer memory? Otherwise the best idea I have is copy you saved files else where, reinstall the game with the 2 disc option, and then put your saved files back in the appropriate folder.
man we really need someone with computer expertise to play this game, love it, and stay on this forum forever to help out people.