No Connectivity

Problem: Background - A customer running XP Pro-SP2, with DSL installed at home could not access the internet; the LAN connection was reporting "Limited or No Connectivity". After much effort, and switching DSL modems, the DSL technician asked the customer to back out SP2; he was concerned that SP2 was not only reporting the "Limited or No Connectivity", but also causing it. This didn't solve the problem so the customer decided to cancel DSL service and try cable. Unfortunately, the cable technician couldn't gain access to the internet with the customer's machine, either. So, the machine was left running XP Pro with SP1 installed and no internet access.
Enter yours truly.....
The few times I had seen the "Limited or No Connectivity" situation before turned out to be network hardware-related (bad wires or ports), so I really didn't believe the machine or software really had a problem. But, since this was a brand new cable installation I gave the cable company the benefit of the doubt; the modem seemed to indicate no problem communicating with the cable service.
I tried:

1. Re-installing SP2 - No help
2. Running an XP Setup Repair - No help
3. Changing from the cable company-supplied USB connection to a known good Ethernet cable - No help
4. Installing another copy of TCP/IP - I thought this worked because the "Limited or No Connectivity" indicator went away and the connection seemed to be good, BUT we still couldn't get to the internet (throughout the process ipconfig showed the dreaded 169 address, meaning no address was being assigned by DHCP). Anyhow, no help
5. Moving the machine to another location with a known good cable connection - No help
6. Installing an SP2-related hotfix reported as a possible solution, and an associated registry fix (see:KB884020 and Reg Fix) - Sorry, but this didn't help either.

Solution: I had run out of time and patience and so had the customer, so we saved all her data and started from scratch. I wiped the hard drive, re-installed XP, then SP2 and all the current Windows updates. This allowed us to get on the network and back in business.

I hate to get to this point; we really didn't "solve" anything. But, sometimes you pass the point of deminishing returns and find that it's less expensive and less trouble to just rebuild the system. At least we didn't lose anything but our precious time..... :o)