I think Kull means to create a boot menu and install a 2nd OS on the system, and that 2nd OS would be used only for playing Rome. After you've played the game, you'd alt-tab to desktop (leaving the game still running) and put the computer on standby.

It sounds like it could work, but I'm not sure if you can actually use the boot menu while coming out of hibernation. Still, this option is worth checking - it's surely cheaper than using VM software.