What operating system do you have? (eg windows 7 x64 or windows Vista x32)
And what DirectX have you installed on your PC?
You can find that out by doing the following:
1. Press and hold the windows button (next to your alt button on the left of your keyboard) and R at the same time.
2. Something should pop up. Type in "dxdiag" (without the quotes) and hit enter.
3. Wait for it to load, and you should get a list of specs, with on the bottom (of the first tab) "DirectX version:".