If your important application hasn't yet been made compatible with Mac OS 10.7+, but a comparable windows version exists, then it may be worth looking at a system called WINE.
WINE is very clever - your Mac thinks it is running a standard x-windows application, and WINE sends the windows ".exe" executable the kind of signals that it would expect from the windows OS. You do not need to install windows at all, the windows application runs pretty much "natively".
There are lots of flavours of WINE, some commercial (like "crossover"), some are mac-friendly pakaged up and free to use (like "winebottler") or you can compile your own from the open-source original (http://www.winehq.org/)
See Julian if you want to know more