Can't browse beyond first site

Using IE6, I can connect to any website, but any attempt to browse beyond the first site gives me the famous "This page cannot be displayed" message. I'm running XP Home, and this is a recent problem. Any suggestions?

Early on, I suspected my ISP's DNS, but eliminated that by connecting using another computer, and it works fine. So, it's me, not the ISP!

Deleted all my temporary internet files, but that didn't help.

Same thing happens with Firefox.

 

I now believe the problem was due to a virus: backdoor.irc.bot

The virus was able to turn Symantec off. When I forced it back on and ran a scan, it found the infected file. After that file was quarantined, things seem to be back to normal.