Your browser has been hijacked, probably by some piece of scumware editing the HOSTS file. The HOSTS file lives in C:\WINDOWS\SYSTEM32\DRIVERS\ETC (substitute WINNT for WINDOWS if you're using Windows 2000).

Open HOSTS with Notepad. (CAUTION: NEVER open HOSTS with Word Pad, Word, WordPerfect or any word processor that adds formatting to the file. Strictly text editors only.) You will see a lot of lines of text preceeded by #, which are comment lines. The only line without a # at the beginning should be:

127.0.0.1 localhost

If there's anything else in the HOSTS file other than the # lines and that one line, delete them all and save the file. Once you've done that, set its properties to Read Only. That won't stop all hijackers, but it will slow most of them down.