Lost Internet Connectivity

I'm the unofficial go-to guy for computer problems at work, and I have
a problem that I think is beyond me.

This morning, my boss turned on his computer, and had no internet.
Nothing had been touched since he last used the computer, when
everything worked fine. No one had even been in his office, except
when an employee came in to clean. However, the network worked -- he
could send/receive files with other computers on the network -- but
neither Intenet Explorer nor AOL could connect to the internet. At
first, this was the only apparent error, but after I started
switching NICs, I got something else. The Local Area Connection
became labeled "limited or no connectivity" and when I
clicked "repair" under the support tab of the Local Area
Connection Status window, it failed because the IP address could not
be renewed. As you can see below, I tried a lot of things, and
nothing has worked. I've ran out of ideas. Please, any
suggestions???

What doesn't work:
Update programs can't connect to the internet
AOL can't connect to the internet
IE can't connect to the internet
Will not successfully ping 192.168.0.1 or 192.168.1.0
ipconfig does not list a default gateway address
ipconfig does not list a DNS suffix

[b:7b4a71812f]What works:[/b:7b4a71812f]
The network -- all computers that can typically be accessed by this
computer can be accessed, and can access this computer
Other computers on the network can access the internet
All hardware seems to be working correctly
Will successfully ping IP address of computer (169.254.?.?)
Will successfully ping 127.0.0.1

[b:7b4a71812f]What I've tried:[/b:7b4a71812f]
Ran Spykiller
Ran Ad-Aware SE Personal (latest definitions)
Ran Norton Anti-virus 2002 (latest definitions)
Did a System Restore to 3-2-06 (when internet worked)
Unplugged/replugged power to switch
Switched NIC with known working NIC
Put NIC in known working PCI slot
Plugged PC into known working outlet/cable
Computer has been rebooted multiple times throughout all this
(obviously)

[b:7b4a71812f]Description of network[/b:7b4a71812f]
Cable broadband (I believe)
All of the computers (9) are plugged into a Linksys 16-port switch
(sorry, no model #). I have no idea what the switch is plugged into.
There is a D-link wireless router (sorry, no model #) plugged into
the switch, which works fine.

[b:7b4a71812f]The OS(s) used[/b:7b4a71812f]
The computer with the problems is running XP PRO SP2
Five other computers are running the same
One computer is running XP PRO SP1
Two computers are running Windows 98 (1 SE, 1 Regular (I believe))
(One of these lost internet connectivity last week, not sure if this
is related)

Please, any ideas would be appreciated.