In addition to using Swat-It I recommend using a programme called Ad-aware - This is free.
http://www.lavasoft.deYou may also want to use another programme called Guard-IE with your Internet Explorer (if you use I.E.) This programme will show you which web pages have bugs on them, tells you how many cookies have gone on to your computer & blocks those annoying popup adverts when visiting some sites (just to name a few things it does)
Unfortunately this programme only has a 20 day free trial .. after that you need to purchase it to continue all it's functions that it does.
http://www.guardwall.com/en/index.asp 
