Browser Hijacker, Spyware and adware is usually the caused of it. Have you tried running an anti-spyware scan?

Scan the system using Ad-aware SE and Spybot Search & Destroy if you haven't use them.

Ad-aware SE - http://www.majorgeeks.com/download506.html
Spybot-Search & Destroy - http://www.safer-networking.org/index.php?page=mirrors

Update the programs prior running a scan.
Reboot after using the programs.

Next, download CWShredder from http://www.intermute.com/spysubtract/cwshredder_download.html
After installing it, run CWShredder.exe and let if fix what it finds.

Reboot when done.

If no luck to remove the browser hijacker or spyware, download HijackThis from http://www.spywareinfo.com/~merijn/files/HijackThis.exe

Where to put and how to use HijackThis:

It is important that you will run HijackThis.exe in its own folder so the backup files that HijackThis file will create will not be accidentally deleted.

Open 'My Computer', then double-click to open C:\ (or the drive letter that your Windows is installed)
In the menu bar, click File-->New-->Folder.
That will create a folder named New Folder, which you can rename to "HJT" or "HijackThis". Now you have C:\HJT\ or C:\HijackThis\ folder. Put your HijackThis.exe there, and double click to run it.

Click 'Scan' button. Click 'Save log' button. Save the 'hijackthis.log' in your desktop. Copy and paste the content of 'hijackthis.log' and post it in any forums that offer HijackThis analysis. You can find these forums at http://www.a-sap.org/

Note: Most of the entries in HijackThis are harmless. Do not try to fix anything if you are not sure what to do. Seek advise from HijackThis experts that you will find in forums listed in http://www.a-sap.org/ for proper fixes.