I run a small network as part of my job.
Last month I had two seemingly separate problems with my network (one server running MS SBS 2003 and 9 computers running either XP or 7).
First - I had three computers experience this same thing. The computer could not connect to the network because the driver for the NIC was either missing or corrupted. NOTHING i tried worked. I tried putting in new cards and re-installing software, changing registry settings, restoring the computers to earlier (happier) days. After I lost the third I was going crazy.

Then the second problem all of the other computers lost the connection to the network. This seemed different since the NIC drivers were still present and 'seemed' to be working just fine in the 'working' computers. The server was just fine.

I finally had to call in a network guy to fix the second problem figuring that the first was just some sort of nightmare mal-ware or virus. In under 15 minutes he figured out that another device on the network (turned out to be a WAP) went rouge and was giving out competing IP addresses to the 'working' computers. We unplugged that, restarted the computers and suddenly they were working again.

BUT when I re-connected the three computers that had no NIC drivers (actually it was down to two at this point - I had killed one trying to save it) these were suddenly working just fine. I still do not understand how a WAP could do that and why it would make a computer to not recognize it's own drivers but after a month all the computers are still working.

So look for devices that may be attempting to give your computer conflicting IP addresses.