No introduction needed for this game I guess.
Cheapest I can find elsewhere is £33 from Shopto.
Use the promo code: Savemore to get a 25% discount on the listed price which brings it down to £29.96.
You will then get a product key which you activate on Steam and you get the Steam version of the game. No need to mess around with D2D downloader software.
Call of Duty®: Black Ops Download Minimum System Requirements
* OS: Windows® Vista / XP / 7
* Processor:Intel® Core™2 Duo E6600 or AMD Phenom™ X3 8750 or better
* Memory: 2GB
* Video: Shader 3.0 or better 256MB NVIDIA® GeForce® 8600GT / ATI Radeon® X1950Pro or better
* Sound: DirectX 9.0c-compatible
* DirectX: 9.0c