Fully using Windows performances?

Hi there, I have a laptop running on windows, and I was hesitating to put this in the video games category, but the performances is totally weird for everything I do out of games.

I'll explain a bit more.

From the website "Can You Run It?", of System Requirements Lab, I saw that my computer's performances are high, but yet, I cannot run most of the games, and if I do, they lag like hell. Just to browse folders is a pain.

Here's what I've got.

CPU Speed : 2.0 GHz
RAM : 2.5 GB
OS : Microsoft Windows Vista Home Premium Edition Service Pack 2 (build 6002), 32-bit
Video Card : ATI Mobility Radeon HD 3470

Though all this, my computer can run a requiring game like Magicka (Though it is slow), but not even open correctly a game like Terraria, though both appears as perfectly runnable on "Can You Run It?"

My question is : "Is there a way to actually increase my laptop's performances, or to fully use its capacities without having to buy things to make it better?"

Thank you in advance.