This is one of those "power cycle the building" issues. Every computer remembers it's IP, and the router has no clue which ones are in use. Every time you power cycle the computer, the router hands it an address, which one of the other computers is still using, so it refuses to use anything. Reboot the building, and you're all set.

Easier said than done. To accomplish the same thing, unplug all the switches, let them rest long enough that all computers have dropped their links, then plug them back in.

Matthew