Firefox ad blocker – Adblock plus – easy install. Works well.
Google Chrome ad blocker -Adsweep.org – follow the instructions, it will only take a minute or two to copy and paste what you need.
IE – download and run IE8, turn on the “in private” browsing thinger.
For all 3 – download and install the hosts file from MVPS.org. This maintains a list of bad guys and prevents your computer from having any traffic going there. Make a calendar appointment to update this every few months or so.