One of the reviews on your Graphic Card...
Oooh, new GPUs from AMD - however these ones aren't going to be breaking any performance records, they are both priced below $60.
The Radeon HD 4350 and 4550 are slotted in between integrated graphics and the set of hardware we took a look at recently in our Radeon 4670 review. The Radeon HD 4550 will run you around $45 - $55, while the Radeon HD 4350 will be priced at $39. Generally speaking, if you're a gamer you're not spending any less than $150 for a graphics card - so these GPUs are mostly for enabling hardware Blu-ray acceleration or providing a boost in performance over games running on integrated graphics.
Now, a review on your CPU...
The Intel Pentium E5700 is a low-end CPU
CoD4 is a very optimized game but therein lies your problem, you should consider yourself lucky that you could play on Medium.
Try playing Rainbow Six Vegas on your pc and you'll see what I am saying about optimized games.