It's the return of Nanami Madobe! She's the OS character (née OS-tan) for Windows 7. OS-tan is an anime personification of computer operating systems; they're usually created online by fans. Nanami Madobe, however, was the first official OS-tan.
Windows 7 was Windows 7 in Japan—it wasn't any different other than being in Japanese. However, back when Windows 7 was launched in Japan, the first 7,777 copies of Windows 7 Ultimate DSP editions included special Nanami Madobe wallpaper. In Japan, Nanami was used to promote Windows 7, and now that the release of Windows 8 is right around the corner, she's getting a send off with an online card battle game on Microsoft's Japanese site. In the game, you click on the corresponding card to defeat the "monster".
As sister site Kotaku Japan points out, this is probably a way to send off the character in style—so we don't forget her! Ha, like that's possible.
殿！ 殿のご乱心じゃあ！Window7イメージキャラクター窓辺ななみがカードゲームに [Kotaku Japan]