it could be 1 of three things, a virus(es) , spyware or adware!
Use norton or McAfee to get rid of viruses
use spybot to gte rid of syware (download it from www.download.com
use adware 6.0. to get rid of adrware (download it from www.lavasoft.com
that should sort the problem out, if it doesnt reply to this message