Unlike Microsoft, Sony never bothered to make PC drivers for their PlayStation 3 controllers; but some Japanese programmer has bridged that fjord for you.
This Geocities page (wait, what?) has the download links, and this forum has English instructions on how to get it working. It works in XP/Vista, and theoretically should work on Windows 7 as well, but who the F knows. [Crave Asia via 3g iPhone Hacks via CNET]