Umm... Gimp? Photoshop?
The thing is that there is no single piece of software that will work on both systems- the reason for this is that they are running DIFFERENT OPERATING SYSTEMS. This means that Photoshop/Gimp for mac, will not run on a PC, period. You can get both versions, PC and Mac, of both softwares, and run them, but no single piece of software will run on both.
My advice is get Gimp- it's free, and it requires less resources than PS.