This was one of my favourite games on the XBox 360 when it came out and just had to snap it up when I saw it this cheap.
£14.99 down to £3.74. Cheapest I could find it before was around £9.
Prototype 2 is also on offer separately at 75% off £19.99 down to £4.99http://store.steampowered.com/app/115320/
Or you can get the franchise pack that contains Prototype, Prototype 2 and Prototype 2 RADNET access pack (worth £6.99) for £8.74 down from £34.99 (at same link - this is what I bought).
Both games don't need high spec machines.
Processor: Intel® Core™2 Duo 2.6 GHz or AMD Athlon™ 64 X2 4000+ or better
Memory: Vista 2 GB RAM / XP 1 GB RAM
Hard Disk Space: 8 GB
Graphics: All NVIDIA® GeForce® 7800 GT 256 MB and better chipsets. All ATI Radeon™ X1800 256 MB and better chipsets
Processor: Intel Core 2 Duo 2.6GHz, AMD Phenom X3 8750
Memory: 2 GB RAM
Hard Disk Space: 10 GB
Video Card: NVIDIA GeForce 8800 GT with 512 MB RAM, ATI Radeon HD 4850 with 512 MB RAM