Recently Microsoft has added a  new section to the  Games+Marketplace section at the Xbox site. The new PC  games section includes game demos, videos, add-ons and  direct downloads after buying the game online through an  Xbox Live account. Users will just have to download  a Games for Windows Marketplace Client which is  available at the Xbox site here

All you have to do after downloading this client is either buy a game or try a demo…

Although you’ll need a valid Xbox Live account for buying full games and add-ons online with Microsoft Points, you can simply use your Hotmail account for downloading game demos and videos.