Thanks everyone. There were no errors detected when I ran dxdiag, Jean.
Where did you find those numbers, Hethwell? Like I said, my card says it has DirectX 11.
I would rather not get a new card just to get by for now since I hope to upgrade to a new pc soon, but I guess I might have to if Shader Model 5 is out of reach for this one.
Here is my driver info for my GTX 260:
Main Driver: nvd3dumx.dll,nvwgf2umx.dll,nvwgf2
Driver version: 9.18.13.4195
Date: 1/29/16 6:08:42