Games for Windows live have today put pc games for sale on their client, to act as an incentive to get people to buy of there they are giving away a free Live arcade game Tinker away.
All you need to do is download the client (here >http://www.microsoft.com/games/en-us/live/Pages/livejoin.aspx)
and sign in either with an Xbox live gamertag or if you have an hotmail or other Microsoft email address you can sign up for free and create your own gamertag or just create an email address from hotmail then a gamertag. When you have signed in click on tinker and pay by Microsoft points (it says free though) and it appears in your download que allowing you to download it whenever you to (its only 60mbs).
This game is full and free and even has 200GS attached to it.
EDIT: for some reason the link isn't working here it is copy and paste it, http://www.microsoft.com/games/en-us/Games/Pages/tinker.aspx