You have two options. The 1st is what Alf suggested which is what i currently use, however i wouldn't suggest it unless you have a spare windows cd key lying around.
The 2nd way which is what i used to use was a virtual desktop called VmwareFusion. This option is more suited if you don't plan on using windows as your primary computer and just for smallscale programs. (there are quite afew youtube videos showing how to install and use this so it shouldn't be to hard to find any tutorials)
Basically lets you run windows on your mac as a program which has the benifit of being able to drag and drop files from your virtual machine windows straight to your mac desktop or viceversa. The downside however is it is fairly laggy and not ideal to be used for things such as games as even just the graal client is laggy. If you intend on using it just for graalshop, images and sounds its perfect for you.
Hope this helps
