Random words on webpages presenting as relevant html links, but they are advertising!

Ok, so my PC is running quite slowly (Windows XP), but my biggest concern at the moment is this thing that there are words on websites presenting as html links for to something relevant to the website you're viewing like a word like 'Required' is highlighted or overscored so you think it is a releveant link but when you click on it- a new window pops up with advertising usually for a competition to win an i-pad or similar. Is there any Norton product that cn get rid of this?

I use firefox as my browser, but I have a feeling when I was using chrome I may have copped this (chrome seems very insecure to me)

 

If anyone can help - many thanks (I did just use Norton's NPE) , but it didn't get rid of it.

 

Paul