As mentioned above, you may have a virus. If this is the case, try downloading adaware se, spybot search and destroy, spywareblaster. The first two are scanners which will look on your machine for known malware and attempt to remove any files they find. The second is more of a protection program that blocks sites known to be harmful.
After you first download and install them, you should immediately update each program and then run adaware and spybot. Delete anything they find. If problems persist, download hijack this and post your log to this forum for help:
All of these programs are free. I have used them for years and they do alright. The people at the forum are a great resource, helpful, and friendly.
adaware - http://www.lavasoftusa.com/
spybot - http://www.safer-networking.org/en/download/
spywareblaster - http://www.javacoolsoftware.com/spywareblaster.html