Affinity setting

I am running Streets and Trips 2005 on a Dell with dual core processors. My OS is Vista Home Premium. Streets and Trips runs "mostly" OK. The one problem I have is that I can't use the "Measure Distance" tool. After drawing a short line on the map, the program crashes. After reading that some programs run better on one processor, I followed the instuctions to set the "affinity" to one processor for S&T and tried the "Measure Distance" tool again. Lo and behold, it worked fine. However, each time I close the program and reopen it, it reverts back to two processors. Do I have to change the affinity setting each time I use the program?