Another vote for Daemon Tools. Been using it for a few years now. They have come out with a Lite version, which gives you all the basics and it is freeware. I think you have a limit of creating only a few virtual drives, but I can't recall for sure. I am running Windows 7 Ultimate x64 and have yet to have any issues. Easy install and config. I disable the auto-start via msconfig and run on-demand for the few times that I use it.