Have a Good internet conection but cannot open, IE, Firefox or Thunderbird

I recently had Microsoft Australia fix a problem with opening Office 2007. The problem was fixed but now, even though I have a very good internet connection, I cannot get on-line with IE, Firefox or Thunderbird. I am running Windows 7 Home Premium, 1TB HDD and 8 gig RAM. I have Nortons Internet Security.

Regards

Ian BRIGGS