if its simple just down load adaware and avg free firefox and trillion.
first goto add and remove programs n see if theres any funky stuff in there if there is remove it. then hit control alt delete if its a xp machine hit the processes tab now look thru there for any funky stuff like maybe some thing that says "Iexplorer" if theres something you question simply wright exactly the image name in a search engine like yahoo and see what kind of search results com up then run both adaware and avg free home addition in safe mode n see what happens.