Here is some more about this:
[webmasterworld.com...]
Any chance you have spyware/adware? Or have you recently installed any helper applications? What browser(s) are affected?
Norton Internet Security 2004 (and others?) seem to block ads with URLs which include words like 'ad', 'ads', 'adwords', etc.
So my guess is that if you have tracking URLs which include things such as source=adwords, you'd be better off with source=aw instead.
Might be worth checking with Norton, to understand the nuances. But this seems to hold true.
Hope this isn't old news. ;)
AWA