Graphics problems

I'm suddenly having some problems with graphics on my PC. This is one we had in our office for a couple of years, and it worked with Aecosim without any problems. Might be that I changed the graphics card from AMD to Nvidia a while ago, found out that I had some AMD graphics drivers installed as well, which I removed now. Haven't experienced any problems in other programs.

The problem are the lines in the model, they are not real. You can't click on them either.

Card: Nvidia 8800GT, latest drivers

Any ideas?