The problem with Windows XP is that they can't use more than 3GB memory, unless you install x64 version, which proved to be very unstable - it isn't a surprise most people didn't bother to switch from 32-bit version. The other problem is that it wasn't designed to use more than 2 CPUs (be it cores or standalone processors), it doesn't use memory as HDD cache, while Vista and Win7 use almost the same design as Unix/Linux, which has proven very effective over the years. Agreed, WinXP is a good OS, but not if you want to fully use today's hardware.
Back on topic: If one reads the Garmin 430 manual thourougly, I think you don't need the simulator at all. Surely, it's an advantage, but if you can't find PC with Windows XP to run it, all you really need to do is use as much functions of 430 as you can on two or three flights or let your buddy fly while you "play" with the GPS and you'll learn at least the basic functions very fast.