Doom Builder 2 error

Whenever I try to open a map or create a new one in Doom Builder 2, I get the following error:

Unable to initialize the Direct3D video device. Another application may
have taken exclusive mode on this video device or the device does not
support Direct3D at all.

I've seen various threads for this on many different sites, but none of the methods seem to work for me. I'm using a 32-bit Windows 7. I can provide any additional information you need.

EDIT: I think that Direct3D is not working at all (WARNING: Some non-Doom-related stuff is present in the following sentence): my screensaver isn't working (and instead shows a "bounce-off-the-edges" message box involving a Direct3D error), GZDoom gives me an error whenever I try to run it (though strangely, it doesn't say anything about Direct3D), Minecraft won't even start up, and Team Fortress 2 gives me an error right after the Valve logo is displayed.

Does Windows 7 run Aero? Do you have proper GPU drivers installed? What's your hardware. GZDoom uses OpenGL and not D3D (I think Minecraft, too).

I believe GZDoom uses OpenGL. If both that and DirectX don't work, then the problem is likely with your video driver or hardware. Make sure you have a 3D graphics accelerator card/chip and that you have the most recent (official) drivers installed for it.

Where can I find the drivers I need?

Know the manafacturer of your 3D graphics accelerator card/chip, go to their website and get the drivers that match with your hardware type (by product type code). If you are unsure about that, ask someone close to you (friend/family) who knows about these things to do it for you.

I found a list of drivers for my computer, but which ones should I download?

EDIT: Scratch that, I downloaded the right driver and everything's working fine now. Thank you!