on some websites you may get these pop ups saying you need to install a plugin for your browser in order to view the page correctly.if its macromedia/shockwave or flash etc then you should be safe enough to download it.
use your head though.
what is the website you are on at the time you get this.some can be dialers on dodgy sites.even clicking no may in fact be accepting without your knowledge.
keep your av up to date.
use spybot,adaware and spywareblaster to get rid of adware or spyware.spywareblaster stops it getting on in the first place.
click here click here click here after installing these,update them and run.remove anything they find.