That is indeed odd.
Did you by chance use the option to install "for me only"? If so, try to uninstall, and re-install to some other folder NOT in program files. Sometimes Designcad wants to write to its program folder, and Vista won't allow it.
When re-installing, try turning off your anti-virus, sometimes (rarely) that screws things up
If that doesn't help, I think I'm out of ideas...