I'm definitely with helgriffen and snowy_guy; it's spyware/malware, that is, programs that were installed by some website to track your web-surfing. Not all these programs are "bad," but those that are can really screw up your system.
First, download and run "Adaware" (free). Go to
http://www.lavasoftusa.com/software/adaware/
My guess is that AdAware will fix the problem, but just to make sure...
Then go to
http://www.pctools.com/registry-mechanic/
for a free, limited registry scan.
You can buy upgraded versions of both programs, but usually the free version works fine. I use both, and run them every week or two. Note that AdAware will remove stored internet passwords unless you specifically tell it not to, but storing passwords is a Bad Idea anyway.
Good luck!