Google "Windows Compatibility Mode", and see if that will allow it to run - a Windows feature that allows software to think it is running on older versions.
You didn't note what version of Real Flight you have, but likely the program is checking to make sure you aren't using a really, really old version of Windows. Unfortunately, it itself is too old to recognize versions of Windows newer than what existed when it came out. Since the version isn't on its list, it reports that it can't run.
If you do decide to upgrade, go over to the Knife Edge site/forums first and do a bit of research. Because even the latest is based on a pretty old graphics engine, there are major brand video cards it no longer works with. If you have a NVidia card, you will be fine.