Well there is Apples Boot camp
Here But you will need a windows OS ( XP or Vista ) to use on.
Other emulators such as Virtual PC and Guest PC whilst you have to pay for them, they are still no good for playing games, as they only emulate a very standard video card, with no 3D acceleration.
Bootcamp allows you to you the actual mac hardware on the windows install.
There is also
parallels/ again it is not free, and you will still need the windows CD for XP or Vista.
To my knowledge there is no free emilator that will use the mac hardware correctly as bootcamp or parallels do.
Bootcamp being free is the better bet for you to test out, whereas parallels is cool as you do not need to restart the mac to get into windows!