Graphics issues in all installments (Vista).

I bought a licensed copy of "HoMM V Gold Edition", installed it, and everything seemed to be working fine, all three parts. Then, in the campaign for the Dark Elves, in the last mission, a script error occurred, and I was kicked out. After some time, I logged in again and saw square shadows. When I entered the town, the picture lagged and slowed down. I exchanged the disc, and the same thing happened again:confused:. I downloaded the latest DirectX, updated my video card drivers, and installed patches, but nothing helps:mad:. Specs: Vista Home Basic Intel Core 2Duo nVidia 7900 512MB 2 GB of RAM Please help, even if it's just with advice; I don't know what to do anymore!!!