There's two methods on the hosts list. While we don't use windows servers (only unix) and our staff mainly use Linux desktops, here's the deal on the windows hosts list:
The host (your web server guys) shoudl be checking that file for any anomilities, however the USERS file can also be affected:
http://en.wikipedia.org/wiki/Hosts_file
Basically the file should be BLANK or at a minimum, known IPs. These are generally used to speed up searches and destinations on the web. Some people edit this file when they are moving sites from one server to antoher and want to test things.
Anyway, the file shoudl be empty. Open the HOSTS file with Notepad and make sure the file doesn't have anything in it. If there's something in it, then esentially what it's doing is trying to reroute you to another location.
If for example it has "yahoo.com" and then an IP number beside it, then that's probably fraud. Delete the line, and let it pick up yahoo on it's own.