I had an interesting network problem today. I arrived at work, plugged my laptop into the power, and then into the network cable, switched it on only to find that I had no network connectivity. After rebooting twice, and discussing it with my colleague sitting next to me who has pretty much an identical laptop, connected to the same router, with no such networking problems, I did a bit of poking around.
I have a latitude D820 running Windows Vista, and the Dell Quickset power utility. One feature of quickset is that it disables the internal network card if you are not using it and you are on battery power, to get a few more minutes of battery life. It turns out that it must be a bit buggy as this turned out to be my problem. It had disabled my network card from the night before, and admittedly I had suspended my session, but then this morning, even after plugging into the power at work, and after rebooting my system twice, my network card still failed to respond. I fixed it by explicitly setting in the quickset UI to "always activate on battery".