Windows 7 doesnt run the games i got

This might not be the right place to put this...

I just put together a new computer from parts a friend was using. He just upgraded and gave me all his old parts.
its a pretty basic setup right now that I'd like to upgrade over the next couple of weeks, but im at a crossroads here.

He told me this is the machine he played starcraft 2 on, and modern warfare 2.

I went out and bought both games, and the weirdest thing is - they really dont work at all. Ive seen him play them at his house, but I dunno whats goin on here.

the cpu is P4 2.39 Ghz. GPU is Geforce 7600GT with 512mb memory. I have 2 gigs DDR ram (4 512mb sticks).

Im running windows 7. Now, the only things im not certain of is if he was running XP, also if he had 4 gigs ram instead of 2.
Would this really make the difference though? Would my specs with doubled ram really even run these games properly?
The more i look around on the net, I have a pretty low end gaming system, if i can even call it that.

I can run starcraft 2 at the lowest possible settings, but every now and then it tells me to lower the settings and gets choppy as hell.
Modern warfare 2 runs, but the frames move an inch every second
What do you guys here recommend?

cross-post.