Sounds like you have some unwanted spyware or similar on your pc. Have you ran a virus scan and is your anti virus up to date?
I use a couple of spyware programs for hunting out problems.
http://www.lavasoft.de/software/adaware/
http://www.download.com/3000-2144-10122137.html
Give that a try and it's also worth emptying your internet file and java files regularly too.