After you run SquareSteelBar's program, disconect from the internet, startup in safe mode, turn off all antivirus and anti spyware programs that may still be active, go to windows explorer, find installshield folder and delete the regestry key that refers to SH3. Reboot, stay off the internet, disable all antivirus and anti spyware, and try again to install SH3.
If this doesn't work, shoot the computer.