NSLookup works, but DNS lookup fails everywhere else

I have one computer in my office that's having strange connectivity problems. If I try to go to, say, "google.com" in IE, it will fail to find the server. If I do an "nslookup google.com," the DNS lookup works fine, and it gets an IP address... and if I then put the IP address into IE, it connects just fine.

It's not just IE... other programs have trouble too. nslookup, however, works just fine every time.

I don't see anything relevant in event viewer. ipconfig /all shows the DNS servers and everything else correctly. I've tried ipconfig /flushdns and ipconfig /registerdns, but neither helps. Of course I've tried rebooting, too. Other computers on the network with the same configuration work fine. This one just started malfunctioning a few days ago.
Joshua-JAsked:
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AManouxCommented:
Try installing a different browser, like FireFox, or Google's Chrome, on the computer having the issue and see if you run into the same problem.
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Joshua-JAuthor Commented:
No need to try other browsers, I've already established that I have the problem in programs besides web browsers too. Telnet, for example. Even joining the computer to a domain: I dropped the computer from the domain when it started having problems (because the first sign of the problem was drive mapping problems and logon script issues) and when I tried to re-connect to the domain, I got a message that it couldn't find a domain controller when it looked up the relevant SRV record in dns. However, I can look up the same SRV record using nslookup (setting type=srv) and it works just fine.
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AManouxCommented:
What if you specify an external DNS server IP address in the TCP/IP settings of the computer having the problem? For example, 4.2.2.1.  Does that work?

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SteveNetwork ManagerCommented:
do you have a proxy server that is caching your outbound requests ?  (IIS or ISA server?).. ive seen this exact same scenario when there is a misconfigured ISA server on a network..

have you checked for viruses / trojans ? checked your hosts file ?
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Joshua-JAuthor Commented:
Tried switching the DNS server to 4.2.2.1. Same problem:

IPconfig still works using that server.
IE still can't resolve domain names using that server.
telnet can't resolve either. (Example: telnet to google.com port 80 fails; telnet to 209.85.171.99 port 80 works).
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brookslawCommented:
Try installing Wireshark and tracing what is being looked up when the apps do a DNS lookup.  Kaspersky antivirus did something similar to me at one point, although it only altered some requests.

WireShark can be found here:
http://www.wireshark.org
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SteveNetwork ManagerCommented:
have you turned off your firewall ? i know that with products like zonealarm etc if you dont specifically allow the apps to connect then it'll deny them.. try turning them all off and testing..

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AManouxCommented:
Any change XP SP3 was attempted to be pushed to the computer through Windows Updates or Group Policy?  I've seen some posts with people that had similiar DNS issues to yours when SP3 did not install correctly.  
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Joshua-JAuthor Commented:
I installed NetMon (similar to wireshark) and found that most programs weren't even doing dns lookups at all, which explains why they weren't finding anything. IE was just doing a netbios name lookup and then failing, and I think other programs were doing the same thing.

An XP SP3 upgrade had been attempted (by a user who shouldn't have had admin rights... oops) and had failed due to McAfee Antivirus. It wasn't the first thing I looked for, though, because: a) That was about a week before the problem started; b) the SP3 update also failed on several other systems, none of which exhibited any problems at all.

However, I fixed the SP3 upgrade anyway (by running the upgrade in safe mode) and it fixed the problem, so we're done.
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Joshua-JAuthor Commented:
Points go to mostly to AManoux who suggested the SP3 upgrade (which fixed it) but also to brookslaw who suggested an essential trouble-shooting tool that will likely help other users with this symptom.
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