to CF
It's not clear whether this PC is wired or wireless. Is there a working PC wired in to the router? If so have you checked you are still getting a valid IP address from the modem? Can you check the DHCP table on the router? Is your wireless secure or has someone in the area nicked one of your DHCP addresses?
HTH for starters.
BTW, if the PC is wireless SP2 should make life easier!