Try to find a file called "hosts" on your computer (it has no extension, it is just called hosts).
In Windows XP it is in a folder called C:\windows\system32\drivers\etc (etc is part of the path name)
On most home computers this should have very little in it, but sometimes rogue programs update this file to try make you go to their web sites.
The only entry should be:
127.0.0.1 localhost
Have a look in yours and see if there is anything else in there.
Read more here, particularly the section on hijacking.
http://en.wikipedia.org/wiki/Hosts_file