Multiple Games don't work

I tried browsing the forum for something to help me, but nothing seemed to
fit my problem.

I've got an HP Media Center PC with Vista pre-loaded.
-2gb ddr ram
-nVidia GeForce 7500 LE (256mb)
-E6400 Intel Core 2 Duo

And yet games like Doom 3, Counter-Strike Source and Crysis can't run on my
PC faster than 10 FPS on their lowest settings.

Please tell me why my5 year olf P4 2gHz, 1gb SDRAM, GeForce 2 (128mb) with
XP can run all of those fine and why my 1 year old PC cannot- and what I can
do to fix this besides changing my OS?

Thanks for any help, will be deeply appreciated.