Have you scanned your comp for malware?
If not, suggest you do that with one of the programs in links below.http://www.superantispyware.com/http://www.malwarebytes.org/mbam.php
Check in your browser addons for listings such as Ask Toolbar.
Without knowing what program is attempting to change your search settings is a problem.
Have you checked the settings for your preferred search engine? If not, do that. Remove all others
if you can.