Uninstall the grapics card completely, including all drivers & Software (remember to use onboard graphics port after this so you can actually see what you're doing) and then put it all back on again including all updates from MS. DX is unusually reliable for Microsoft so my educated guess is a corruption in the card or its software.
If that doesn't work then you may need to have a deeper look at the system - I have used windows debugger bery succesfully in the past (Google for windbgr which is a free download from Microsoft) but you will need to do some reading before you use it. Otherwise it's avg updates, system restores & various other nuclear options, but I would bet on the card and it's drivers/software.