*g*.
Sorry I just couldn't restrain myself.
But here's the reason why:
Programs that end in EXE are programs written in a certain language, such as C or C++ and then compiled for a processor of x86 architecture. Usually these programs have a GUI, which requiers a certain engine to run on. Windows has an own engine (while linux has X11 and macs can handle X11 too) so these programs simply include zeros and ones that your PowerPC won't understand - to put it simply.
Believe me, if there were a way to convert programs like that, Switching wouldn't even be an issue anymore...
Besides, ther is bound to be an alternative. In fact, I'm trying to find certain software for windows, which as far as I know run only on macs!
And you can use virtualPC if it's a small program.