How do I solve "server not found" problems?

Thanks for your reply. I tried as best I could to follow your instructions. I say tried because what came up did not correspond exactly. In any case, it did not help. If you want me to scan some of the pages that come up so you can see what they say, let me know. Otherwise, I will try to describe below what happened:

Under Configure Proxies, it says "Using a PAC file". Underneath that it says PAC file URL and it has a box where something could be filled in but there is nothing in the box and I wouldn't know what to write there.

When I went into DNS, I added the new numbers you suggested but the computer still seems to revert to the default numbers. There is also a section "Search Domains". Should I add something there?

I tried in the past to use the built in Network Diagnostics, but it reaches an impasse and tells me it cannot solve the problem. There has been no change. I just wanted to mention what it does say: Modem, Modem Settings, Network Settings, and ISP are all green lighted. Internet and Server are yellow (before diagnostics) or red (after diagnostics).

Does it matter what "Location" says under Network? It has filled in something there. I didn't specify it myself.

I do notice that other people have posted on Apple or other forums with this same problem, but I am not clear what is the answer that worked for them if there was one. Any further suggestions?

Thanks again for your reply. I made a new location. No difference.I entered the numbers you suggested in Safari. When I click on Safari from the dock, Safari begins to load for a few seconds then freezes, so it doesn't respond to any URL numbers. I should have mentioned that I have 3 browsers available on the computer: Firefox, Safari and Opera. None are working due to the problem I describe here. All of them used to work before the problem.

There is something that I notice that may be a possible clue. On System Preferences, when I click on Network, a kind of status screen comes up. At the top it says "Sent" and "Received". When the ISP is connected (it does that much but no more), the "Sent" line gets moving bars so the computer is doing something. But the "Received" line remains dead. Does this tell us anything? Could it have anything to do with the built in communications software? I'm sure this is a malware problem, but it certainly is not clear what has been damaged or how to fix it.

The IP that is listed is 216.194.21.170.

FYI, the DNS Servers that list automatically (presumably linked to this free dialup ISP (Metconnect) are 216.194.28.33 and 216.194.28.69 in addition to the 2 additional numbers you asked me to add.

If this is a communcations software problem created by some kind of malware, then is it possible (1) to reload the internal software that came with the computer or (2) try loading some other software especially something I can download for free in case it doesn't work I won't be out the cost? Are there any diagnostics I can try besides that built into the computer which I already mentioned is unable to diagnose the problem? Other suggestions? Thanks.

All I got on Traceroute was those asterisks. I think they would have continued indefinitely. I stopped it after 20 lines. I don't see any comments on the Internet that Metconnect is not working. Metconnect is the free metro NYC dial up service. There is no working contact information for them so I can't ask them anything about my problem. I signed up with them and then entered the sign up information on my computer. As I said, it worked for a long time, maybe a year or more, before it stopped working. I think I tried signing up again, but it didn't solve my problem.

Allow all incoming connections is checked.

Since your last answer, I thought of checking whether my phone line might be the problem or whether the fact that I was using dial up was a problem so I took the computer to another house to try their phone line and their wifi. Same result: server not found. So the problem is definitely in my computer. Any further suggestions?