Microsoft is adding another piece to the gaming puzzle with the forthcoming debut of an on-demand Web store for PC games dubbed Games For Windows Marketplace.

Launching November 15, Microsoft says the Games for Windows Marketplace will offer PC gamers a “robust lineup of games” (about 100 titles at its debut, see a limited list below) and recurring specials such as Deal of the Week.

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It mentions a few publishers on the landing page of the upcoming store, namely Microsoft Game Studios, 2K Games, Capcom, Rockstar and Warner Brothers. In the press release, it also names Square Enix and promises to retail new indie titles in addition to big-name games.

Microsoft promises that the store will be optimized for speed, allowing for rapid obtaining and gameplay with fewer clicks to actual purchase and download. Gamers will be able to search for and download games on a PC, anytime, and redownload games they purchased at some point in the past if need be.

From the looks of it, you’ll need a (free) Xbox LIVE gamertag, Zune tag or Windows Live ID to purchase titles on the Games for Windows Marketplace. The new store also supports Microsoft Points as well as credit card purchasing, both of which are big pluses.

The store will also feature promotions on select games every time they visit the Marketplace, combined with the “Deal of The Week” and other recurring and seasonal offers.

To get a taste of what to expect: right now, the Deal of the Week on Gamesforwindows.com is Grand Theft Auto: San Andreas for $3.75 (or 75% off).

Here’s a list of games that will be available (from the press release):

– “Fable: The Lost Chapters”
– “Grand Theft Auto III”
– “Lego Universe”
– “CarneyVale”
– “Dead Rising 2″
– “Lost Planet 2″
– “Max Payne”
– “Deus Ex: Game of the Year Edition”
– “Flight Simulator”
– “Gears of War”
– “Halo”
– “Zoo Tycoon”
– “Fable III”
– “Age of Empires Online”
– “Microsoft Flight”

I’m not sure which is the greater credit to Microsoft: that Windows 7 is the fastest-selling OS of all time, or that the rock-solid XP is still going strong after ten years. Despite numerous warnings and, of course, a great OS update to, a lot of system admins are choosing to stick with the old workhorse for as long as they can, even after it’s no longer supported. Hey, if it ain’t broke, right?

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The study, by Dimensional Research, polled 950 IT professionals, and about half said they’d upgrade by April 2014 (XP’s expiration date), the other half said they’ll stand by their OS. The only trouble with all that is that once XP stops being supported, it’ll start being vulnerable. Security holes discovered in the final patches will remain unaddressed, though I imagine that a non-Microsoft unofficial support community will arise in their absence.

For internal networks, though, or other less-exposed situations, XP will likely stay viable for as long as its file system and basic underpinnings are still supported by newer OSes. Who knows how long that will be, but I don’t think XP will be disappearing from the world any time soon. More info from the study can be found over at Certkingdom.com.