Open this file in Notepad:

C:\WINDOWS\system32\drivers\etc\hosts (the filename is "hosts" without any extension).

After the hashed comments section will be a list of IP addresses mapped to domain names.

There should only be a single entry:

127.0.0.1 localhost

Delete any additional entries you don't recognise.

Then go to Start > Run. In the Open box, enter "ipconfig /flushdns" without the quotation marks.