I have been having this problem on and off for over a year now. Whenever I have an ethernet cable connected to my computer on startup, the graphics driver does not load. This causes everything to be laggy, and I am unable to even watch a video. When there is no ethernet connection, it loads fine and I can connect ethernet after it finishes booting. It used to happen regardless of whether it was connected to LAN.

I've fixed it before by updating and/or downgrading graphics drivers, but the problem keeps coming back. Another thing that might happen is that SLI becomes completely disabled.

Any help would be greatly appreciated.

Specs:
Windows 7 x64 Ultimate
2 GTX 285's in SLI
Asus P6T vanilla mobo
i7 920
6gb ddr3 Dominator GT
950 watt corsair psu