i had the same problem with random underlined word links a year or more ago. it was a bugger to get rid of.
i remember it was a firefox ad-on (still my favourite browser) which had, unwittingly, brought some sort of nasty thing with it. it was nothing much to worry about, just very annoying.
if you use firefox, try starting it in 'safe mode' and see if the problem is still there.
G'day Busby, i think annoying ads is a different prob, some people tolerate them on this site because it might not exist - at least as a free site - without them.
you can always use adblocplus and 'whitelist' certain sites, although i have to admit i haven't whitelisted this site.