So you want to sell advertising on your web site. You go out and sign up with some affiliate network and put the ads on your site and ouilla - your making money every time someone clicks or downloads some software.
Guess what, you may be supporting the proliferation of malware.
Read this:
http://www.eweek.com/article2/0,1895,2013957,00.asp
http://ct.enews.eweek.com/rd/cts?d=186-4422-5-92-283087-513065-0-0-0-1