Is there some way to keep random websites from changing my default search engine?

I am constantly having my default search engine changed to Delta Search, Bing, Yahoo—you name it. I can understand the changes if I downloaded a game from a dubious source (Shockwave.com) but I got a search engine change from Sourceforge.net! I’ve always trusted them.

It wouldn’t be so bad if these search engines were any good, but they are lousy. I’ve never found anything useful with them. How can I get this to stop?