With Microsoft reinventing the Xbox experience later this year with the new One console, they have announced that their PC game marketplace will no longer be sold on Xbox.com after August 22nd. Don’t worry though, as games like GTA IV will still work because Games for Windows LIVE functionality will remain in some form, meaning that whatever system they decide to move to, the backend will have GFWL functionality.
DLC is still up in the air in terms of purchasing, as Microsoft says that it will depend on the publisher.
One thing they do say is that GFWL games are “no longer available from any marketplace.” I assume they mean online, and that if you see a GFWL box in Walmart, you should be able to play it still.
As for Xbox Live Points, they’re retiring that system, so your points will be converted into whatever the value translates to.