With that being said... it should still install on Windows 8 OS. This issue is most commonly seen when person installing is not administrator. Do a cleanup of registries, reboot, right mouse click in setup.exe and run as administrator installing to a location like C:\Solid Edge instead of under program files. There are some known issues with loading ST5 on a Windows 8 machine in the default location. ST6 will fully support Windows 8 which will be released later this year.