There are several things that can cause browser redirection such as malware, browser addons/extensions, registry settings and settings in the Windows Hosts file; the latter being less likely unless you are using WinXP.  

I would highly recommend you run a scan using a free program called Malwarebytes (its free from malewarebytes.org) to make sure you have nothing running locally on your system.   Some Antivirus vendors will not detect Ad-ware programs if the installer of the ad-ware prompted (more like tricked) the user into approving the install.

Next check for any for any suspicious browser add-ons or extension.  If in doubt, disable all the extensions and check to see if you are still being redirected.  

If you are running XP, check the c:\windows\system32\drivers\etc\hosts and lmhosts (there are no extension on these files)  Open both the files in Notepad and look for any lines that contain a name and IP address such as:

248.123.198.203   Google.com    

Normally the hosts file contains only only commented out (proceeded by a #) pieces of text, so if you see any lines that do not begin with #, just insert a # at the beginning of the line and save the file(s).   You can delete the suspicious lines all together if you choose, but I would recommend just a comment # for now.

Check this site for even more details and information

http://www.brighthub.com/internet/google/articles/66090.aspx