There IS some malware on your computer that has modified the hosts file. First tell us whether you're using XP or Vista. If you're using XP, there are some straight forward steps you need to take. If you're using Vista, you need to take some additional steps to take the ownership of the hosts file before editing it. If you're using XP, just follow the following steps. If you're using Vista, tell us and I'll detail the additional steps needed:
Go here:
C:\Windows\system32\drivers\etc
Here you will see a file named "hosts"
Now open a notepad window. Now drag the file called "hosts" onto that window. It will show you some text. The first few lines 16-17 lines have a '#' at their beginning. You are only concerned with lines which DON'T have a '#' at the beginning. There should be these two lines:
127.0.0.1 localhost
::1 localhost
that don't have a '#' at the beginning. These are legitimate and are OK. There will also be other lines which don't have a # at their beginning, put a '#' in front of all those lines to make them inactive. Save the file. This should help. Also, note that this will only help with the redirecting problem, but to fully remove the malware that caused this in the first place, you'll need a good anti virus software.
EDIT: so you're working on Vista. Just do this before opening the notepad window:
go to the hosts file. now right click, select properties. go to the "security" tab. now click on "advanced" at the bottom. go to the owners tab. click on "edit" and confirm UAC request. now in the list that appears, click your user name and click OK multiple times to close all the dialog boxes.
Now run notepad as administrator (for this, type "notepad" in the start menu, now right click on the notepad icon that appears, run as administrator. follow the rest of the steps.
Hope that helps, and sorry I was so late in updating my answer. :)