I have also run in to this problem recently, except the itunes issue, the problem is identical. My laptop would not connect to the net, no matter what we tried. Today I picked up an emachine, and just 2mins ago, the same thing happened.
The only thing we had in common was an external drive we were pulling files from. The external came from a Vista laptop (which also died, we assumed from the hdd though), but also had connectivity issues.
We're banking on this being a virus.
Any help would be appreciated.