I had a good one yesterday. I was hooking a mate's lappy up to Belkin router. The Lappy saw the router & announced it was connected, but continuously cycled through the acquiring network address phase without ever actually connecting.
No show either on the dhcp client list on the router. So, connected but not connected! Tried every trick I could think of, to no avail, then as a last resort, had a look in the services list whereupon I found that dhcp client had been disabled. Switched it back on and bingo!
I asked if he'd turned it off & why & he said that his mate told him to turn off a bunch of services pretty much at random, in order to speed the machine up. He'd noticed a deterioration in performance after he installed autocad & dumped about 60 gig of files on the hard drive, filling it up to about 90%.