It's XP with the Alienware AlienGuise theme Xenomorph added, you can get the pack from here: http://www.alienware.com/intro_pages/invader.aspx
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Intel Core 2 Duo E6300 @ 2.8GHz | Gigabyte EP43-DS3L | GeIL 3GB PC2-6400 | BFG GeForce 8800GT 512MB OC | Seagate 500GB 7200.11 SATA | Western Digital 250GB SE16 SATA | Coolermaster CM690 | Corsair HX520W
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