It pays to look around. Gleaned this tidbit from Forum Click.
In Internet section, StuartW wrote:
Some site you access and download, may contain 'spyware', that logs every site you go to for the purpose of sending you appropriate adverts If you want to check to see if you have any spyware programmes on your computer go to
http://www.lavasoft.de/free.html and download a short programme called ad-aware. This will detect intrusive spyware including the infamous Radiate/Aureate.
I've always defaulted my browser to my providers home page (GTE, now Verizon). Ad-Aware proved that GTE.net was issuing me Doubleclick cookies. Changing the default to Verizon.net stopped the Doubleclick cookie (at least this time). .D