Unwanted pop-up windows while browsing

Am using Win 2k proffesional on my laptop. i have started getting lots of pop-up windows when i start browsing. all these are ads pop-up & is very annoying. how do i get rid of these pop-ups.

am sure there should be some way to to this without using third party tools. please let me know if you have any workaround to get rid of these pop-ups.