Might be worth running a hijack check. There was a DNS stealth attack a few days ago that misdirected all sorts of things to places they'd be better avoiding, and if I recall correctly Microsoft Update and Symantec were both targets of that one.
I can't recall exactly how it was resolved.
Are you on broadband or Dial up. I had a case a few weeks ago at home here where broadband stopped accessing some sites, and the only answer I came up with was to power the local connection box off and force it to reload, once it did that, everything came back clean,