Dxdiag is built into windows and can be very use full. It's a great tool to tell you if your video card is working like it should be. It also tells you if your video card even supports 3d for gaming or is capable of playing movies in the first place. It is available on windows machines. Either under start, run, dxdiag or start dxdiag. It's okay to say yes to check if your drivers are digitally signed.

At the moment I am going over dxdiag in Windows Vista. Notice how if your card is working like it should. You already get to see that it supports direct X 10 and other good stuff about your computer. In Vista simply click on display. If you see no problems found. There is a good chance your video card is working and if your having trouble something else may be causing it.

Now In Windows XP you will see this.

Again, this is already a good sign. However, it is best to make sure and run some tests this time. So go ahead and click on display.

Now run the two tests. The test will show a box floating around and give you a clear idea if your video card is working correctly. Otherwise, the image will not display correct and there is a problem with the card.