Direct2Drive will sell you an account key, as well as allowing you to download the install from their servers. This is usually a less-expensive (and if you have good broadband, faster as well) way of purchasing games, but be aware that yu do not get any discs, media, books, boxes, greeting cards, or fruitcakes from them. Also, there have been some reports of getting invalid keys from them- be sure you keep their contact information handy, in case you need to contact them for a replacement.
