Here's my speccy: http://speccy.piriform.com/results/EGyHCw3wrGHd9S05XcwEkco
I gathered from the nvidia thread that there is a way to force shader 2.0, but I don't know what it is, I've been trying to google it, but I'm not even sure what I should be looking for.
As far as I know, the following relates to my system specifically, and not the majority, but since you mention directX, I'll mention this:
Regarding DirectX, a few days after the problem began I had the idea that might be the culprit and ran dxdiag. When it got to the fullscreen DirectDraw tests it BSOD'd me with a stop error relating to the nvidia driver, and I thought I'd found the problem. First I reinstalled the latest nvidia driver (260.99) using the "clean install" option in the install wizard, and during the uninstall process I got an error stating "The system cannot find the file specified: nvdisp.nvu". Now I *really* thought I'd found the problem, so I continued the install, everything went smoothly, then I ran dxdiag again, this time with no errors. For good measure I reinstalled the newest version of DirectX 9.0 that I could find from Microsoft's website, ran dxdiag, not a problem.
The game ran fine for a few hours then erupted into polygons again >< but it *didn't* expereince a stop error at a place where it had done so twice before.
I recently recovered my system from malware infections with the help of the Malware Removal forum here, and figured maybe my graphics driver was damaged in that process somehow, so at least that's fixed, but it doesn't seem to be related to the root problem, which definitely seems to be an incompatibily issue somewhere.