From a bit of experience i say:
#1 – a virus, install & run avast which is the best free at the minute.
#2 – the way adverts work is that once you have clicked 1 they will always show simirlar to attract your interests.
my advice: run antivirus and clear all cookies & cache and remove any plugins or add ons on your browser (than reinstall the ones you need and trust.)
hatzlocho
ps. if this is a shared computer and it has not got a virus i would strongly reccomend putting a filter on as somebody is obviously using it for inappropriate content.