Welcome to the Org and the wonderful world of Total War takashi.

Your CPU should be fine for STW/WE. I agree with the others that your graphics card is the problem. With an older PC, the other suspects are the DirectX version and the version of Window Media Player. Another consideration is do you have the 1.02 patch installed?

The fact that you can play the AoE II demo doesn't really apply here. The Total War games have a knack for making systems cry Mommy where with other games they say I'm king of the world, i.e., the TW games really give your system a workout.