This could be either a case of Start Page hijacking or a spyware module on your system.
This site has some info that you might find useful: http://www.spywareinfo.com/hijacked.html
I'd certainly suggest that you download and run AdAware, as it will track down many of the changes caused by spyware.
For more information on this topic, visit these other SearchSecurity resources:
Best Web Links: Malware
Ask the Expert: How to remove WNAD
This was first published in March 2002