COD: Modern Warfare 2 LAG?

I just got Cod mw2 today for my new pc. It isnt even a month old yet and is good.

Windows 7 64-bit
Processor: AMD Athlon || X4 620 Processor (4 CPUS), ~2.6 gHz
Memory: 6GB -5.75 Usable
Graphics: NVIDIA GeForce 9100 graphics
With 256MB integrated shared memory and up to 2943MB total available graphics memory as allocated by Windows 7.

It has 640 GB of hard disc space.

I got on COD MW2 and it automatically put my settings on high and a good resolution for my monitor. (19 inch)
I went ingame and it lagged badly. The only way it doesnt lag is if i play on lowest resolution and all quality settings on low. I think my computer can do better? But idk why.

I have a DSL connection and is OK. I also did the Can you run it test for COD MW2 and it got GREAT results.

If you can help me, ty.