WinXP stopped connecting

Fellow WUG'ers,
I'm a bit baffled by this problem. Being no expert at Win machines, I 
beg your assistance.

Wireless connection to the Access Point is shown as
      Wireless Network Connection
      Speed: 11.0 Mbps
      Signal strength: Excellent
      Status: little or no connectivity

But no TCP apps can access the network.

When I open the Wireless Network Connection Status window, I see 
"Limited or no Connectivity: You might not be able to access the 
internet or some network resources. This problem occurred  because 
the network did not assign a network address to the computer." 

Clicking on the Details button shows the IP address as 169.254.74.76. 
(This is a self-assigned IP, no?)

When I click the Repair button, I get see "Windows could not finish 
repairing the problem because the following action cannot be 
completed: Connecting to the wireless network. For assistance, 
contact the person who manages your network."

When my friend first arrived here, we tried to connect the laptop to 
the network, unsuccessfully. After poking around, I found that the 
previous network maven had assigned an IP address manually. I changed 
this to DHCP (and entered the WEP key), and voila!, it connected to 
the 'net. For a half hour. Then no network connectivity, again.

I've checked the TCP/IP Properties and "Obtain an IP Address 
Automatically" is checked.

What can I do to get this computer on the network?

Dell Inspiron 600M (built-in wireless); Windows XP Pro; Apple AirPort 
White (non-Extreme) AP with 40-bit WAP encryption enabled

Thanks,
Dave
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