I'm running it on Vista with no problems.
Find the application, right-click, on the Compatibility tab check the Windows XP Compatible. If that doesn't do it, also go in and check the option to Run as Administrator. I did both at the same time and it works perfectly for me, so I'm not sure if that last step is necessary.
Also check where the program is installed and where you're trying to save your projects - I've had to install some programs in locations other than Program Files because Vista didn't like it when they tried to create their own folders/files in that area. In fact for RMXP, I have the program installed under Program Files but I create all my projects in Documents - I think I started doing that for that very reason.