At this point the definitive diagnosis is not known but an unwanted spyware
or adware pest is leading the list.  You probably have other spyware,
adware, BHOs and such unwanted guests on your system as well.  Get a copy of
AdAware from http://www.lavasoftusa.com, install it, run it and load the
latest updates, then scan your system and clean everything it finds.
Restart the system and run it again.  Keep running, cleaning and restarting
until the system is clean.