I'm having trouble installing project planner 3.1 on windows 7 it says " the version of this file is not compatible with the version of windows you are running. check your computer systems information to see whether you need you need an X86 (32bit) or X64 (64bit) version of the program and then contact the software publisher"
It sounds like your Windows 7 machine has a 64-bit processor which were not available when P3 came out. You have a compatability issues between the software and hardware as P3 was written for a 32-bit processor. Try and right click the install file and choose to run the install in 32-bit mode as an administrator if available.
You can always buy a cheap (under 50$) used laptop/desktop running 32-bit windows xp that would easily have enough power to run P3.