Yup, yet another case of Intel's ever-so-"Extreme" graphics
LordAnki's right. Get a decent graphics card (try a Radeon 9200, they're cheap entry-level graphics cards, but they have enough power to do almost anything at 640x480).
You can try lowering the resolution - when I was using an old laptop (1998, I think) with crappy graphics, I always played games at a 320x240 32bit resolution. That might work:
Common resolutions:
320x200
320x240
512x384
640x480 <-- Default, always try this first
800x600
1024x768
1280x(cough)
1600x1200
Don't even try using 800x600 or beyond, unless it's a rotating cube. On crappier cards, higher resolutions slow everything down greatly.
If you can't upgrade your graphics, then get a new computer.