[mam on edge]

can anyone give me some advice how to blast this parisite into outer space and off my computer for good. it keeps showing up on every page and keeps asking me to download defender antispyware i have to continuosly keep cancelling to get back onto pages viewing. it keeps showing content i do not want my children to look at if they do not cancell. i have Norton 360 downloaded 3 weeks ago, they are asking for a fee of near £67 to try and take these trojans off, after i have just purchaced this product for my sons lap top, i wish i did not download it now, CAN ANYONE PLEASE ADVISE AS I AM NOW BACK ON MY BUDGET again for after Xmas, many thanks.