In reply to Malcolm Tucker's Sweary Aunt:
I spent last night cleaning loads of spyware/adware/other PUPs off my father-in-law's laptop, to stop exactly what you describe.
Open the installed programs list and uninstall things like RadioRage, that hijacks browser traffic and inserts ads. Anything in there you don't recognise, or don't remember installing, Google it and delete it has links describing it as a PUP (potentially unwanted program). Worth re-booting in between deletes, although it's a pain, and you'l usually need to shut down your browser to uninstall them.