Grabbed the system requirements from the official website for you.
Minimum Requirements: OS: Windows Vista Software: Steam Client Processor: 2 GHz Dual Core Memory: 2 GB RAM Hard Drive: 20 GB free Video Memory: 256 MB Video Card: NVIDIA GeForce 8600 GT / ATI Radeon HD 2600 XT or greater Sound Card: DirectX Compatible
Recommended Requirements: OS: Windows 7 Processor: 2 GHz Dual Core (Intel Core 2 Duo 2.4 GHz or Athlon X2 2.7 GHz) Memory: 4 GB RAM Hard Drive: 20 GB free Video Memory: 512+ MB Video Card: NVIDIA GeForce 9000 series / ATI Radeon HD 3000 series or greater Sound Card: DirectX Compatible
Just in case anyone should need clarification since we are close to release.
From the official X-com forum, a pair of posts from a 2K games representative: http://forums.2kgames.com/showthread.php?133786-2k-Greg-please-answer-these-questions&p=1778681#post1778681 "The game is only supported in Vista and above. There's no code in the game that explicitly prevents it from running in Windows XP, and it does have support for DirectX 9, but we're not officially supporting Windows XP."
The reasoning behind the decision not to support XP: "Yeah, the bottom line is that Windows XP is just really old, and is harder and harder to support in modern game development. Keep in mind that no form of Windows XP has even been sold in 3.5 years! And it's been 4 years since it's been sold in retail.
We only have one Windows XP machine here in the office, and XCOM failed to launch on it. But it's an older machine that hasn't been updated in a while, so that doesn't necessarily mean it will fail to run on all XP machines. Again, it's just something we haven't formally tested, so we can't say if it'll work."
This game will NOT run on an XP machine, no matter how up-to-date it is or whether or not it has the best possible gaming hardware, top-end graphics, etc. The O/S is simply incompatible with the way in which the game is coded - as stated above, in the post quoting the developers themselves. They considered that XP was "too old" for it to be worth coding the game in such as way as to explicitly support XP. I personally disagree with this decision: there are MANY PC users out there still running XP, who either cannot afford to (or do not want to) upgrade to Win 7, and Firaxis/2K have probably lost a fair few customers due to failing to support XP. Still, there you have it.
Prior to upgrading my system to Win 7, I saw that XCOM EU was available via Steam at a sale price. Steam's description stated that the game WAS compatible with XP, so I bought it. It downloaded and (apparently) installed correctly. However, upon trying to actually run the game, it merely briefly showed a blank screen then immediately exited to Desktop. Only THEN did I look further into the system specs and find out that the game actually was NOT XP-compatible. And (not boasting here, just saying!), my XP Pro install was 100% in every way: genuine MSoft product, always kept up-to-date on a daily basis, running latest DX version, good gaming hardware with latest drivers installed, regular housekeeping, top-end security software, etc. The game simply does not run on even a perfect and totally up-to-date XP install. The XCOM Forum comment quoted above, i.e. "There's no code in the game that explicitly prevents it from running in Windows XP" may be partly true - i.e., the developers may not have DELIBERATELY coded the game so as to prevent it from running on XP - but it certainly turned out that the game does NOT run on XP, whether deliberately or not. Incidentally, running XP in Administrator Mode (or any other workaround) will not help.
NB : My Win 7 Pro (64-bit) O/S does run the game fine with no problems at all.
Hope this helps - and prevents anyone else from making the mistake I made, and spending a fair bit of money on a game which is not always correctly described when it comes to O/S compatibility!