Hijacked

Many of my searches in IE get hijacked to places like "Coupon Mountain."
etc. When
I do a search, and click on a site, I'm sent to one of these engines instead
of the site I wanted. If I go back to the search, then click on the wanted
site again, I am then sent to the correct site, but many of my first clicks
end up hijacked to one of these unwanted engines.
I do weekely spy cleaning and virus cleanings. Is this problem on the net,
or something on my PC (Vista)? If the latter, anything I can do about it?
I asked this on the IE NG, but got no replies.