Check the hosts file to make sure you're not blocking your own access. If that's the case, time to tweak into submission your anti spyware program.
If nothing there, run Start>cmd > ipconfig /flushdns
If still nothign works, run
www.dnsreport.com (or Alertra Spotcheck) and find the ip address of your site (unless you know it) and add 2 entries to the hosts file for you domain with and without www and that ip address.