Quote from Blizzard.com
"Someone or something, such as a map hack or bot you tried to download has modified your server list. This can be a serious problem, since connecting to a server other than Battle.net can give that server your CD-key, account name, and account password. To reset the Battle.net server list and prevent you from connecting to a non Blizzard Entertainment game server, do the following:
Click on Start, then Run.
Type "regedit" and click Ok.
Click on the + icon beside the "HKey_Local_Machine" branch.
Click on the + icon beside the "Software" branch.
Select the "Battle.net" branch and press the delete key.
Click on the + icon beside the "HKey_Current_User" branch.
Click on the + icon beside the "Software" branch.
Select the "Battle.net" branch and press the delete key.
Programs that claim to alter, hack, or let you cheat at any of the Blizzard Entertainment games may contain harmful code such as Viruses. Viruses and Trojans that are designed to corrupt data or transmit your account and password information are commonly hidden in these programs. The use of these programs is at your own risk. "