...any other web address results in a "page cannot be displayed." No error messages. (I'm writing this on my wife's computer.)

I called my ISP, who had me tweak a few settings. That seemed to cure it, and all was normal for about 3 minutes; then it returned to just being able to get my web broker's site, and nothing besides it. Why just that one site? Why did the fix not stay fixed? I ask you, is 1 web site any way to live? PLease help, thanks.

Data: (Windows ME, I.E. 5.5, same security settings as wife's (i.e. wide open), scanned for spyware, ran AVG for viruses, large cache)