Call of Duty Says good bye to Windows XP it wont be supported due to Blackops DX11 features

so here is the official Black op's Q&A regarding the PC version  

What are the Minimum System Requirements?

OS: Windows Vista SP2 or Windows 7

CPU: Intel Core2 Duo E8200 2.66 GHz or AMD Phenom X3 8750 2.4 GHz

Memory: 2GB for 32-bit OS or 4GB for 64-bit OS

Video Card: Nvidia GeForce 8800GT 512 MB or ATI Radeon HD 3870 512 MB

Can I run the game on Windows XP?

No, sorry. Since we upgraded the engine to use DirectX11, Windows XP is no longer supported. It requires Windows Vista or later.

Will the game actually be playable on my minimum spec machine?

Absolutely. We have tested extensively on a variety of hardware and we are very confident in the performance even at the minimum specs.

What engine is used for Black Ops II?

The Black Ops II engine builds upon the Black Ops engine with a number of improvements including performance optimizations, enhanced graphics, and proprietary anti-cheat technology.

What performance optimizations have you introduced for Black Ops II?

Performance has been a top priority for Black Ops II PC from day one and we have achieved significant improvements. We’ve improved the way the engine handles multiple threads. Additionally, we moved the engine from DX9 to DX11. The DX11 API is leaner than DX9 and requires less CPU time to do the same amount of work. It is important to point out that this benefits the entire range of supported GPUs, not just DX11 hardware. We have also added more “quality vs performance” options than ever before so you can customize your experience to either run faster or to satisfy your taste in eye candy.

You mentioned enhanced graphics for the PC version. Can you tell us more about that?

Black Ops II PC features enhanced lighting, shadows, antialiasing, bloom, depth of field, ambient occlusion, and other enhanced effects that are still in the works. And the game can run at higher resolutions and higher framerates on the PC.

Will there be a framerate cap?

No. Unless you want one. We have included an adjustable FPS cap slider in the game options. The minimum setting is OFF (no cap).

Source   Black ops official forums