From another thread that I didn't want to hijack:

I deal with this question and issue all the time, especially when clients pay for clicks from multiple sources, like AdWords, Homegain, Homes.com and others.

AdWords conversion tracking does absolutely nothing for identifying that any particular user registration came from your PPC ads. You have to have other more sophisticated web site tracking and logging to do that for all of your lead sources, and even that will never be 100% reliable.

Adwords conversion tracking is also only effective on the day the consumer first clicks your ad. It is not persistent. By that, I mean that if a user finds your site via PPC one day, and saves one of your site's pages in their Favorites list, and comes back later but doesn't register for a day or two or three later, then you will never know that their first encounter was from your PPC ad. You may incorrectly assume it was from an organic search or some external advertising medium. You have the same issue for any other purchased clicks.

Likewise, unless you capture and know exactly what page a newly registered user came in on, and that the page they initially came on was purely a PPC landing page that is not indexed by the search engines, or was a landing page dedicated to one specific lead source, then you still do not know for certain whether a newly registered user came from PPC or some other source.

Basically, what you have to do is log each and every IP address that ever comes to your site, capture when that user's first ever visit was, and on their first visit, also log the referring url, and if applicable and available, the SE phrase used. You will still not know exactly who that person is until they eventually register, if they ever do. They may just pick up the phone and call you instead of registering. Then, whenever a new user does register, the registration process needs to check its IP logs to see if the IP address of the new registrant has ever been to the site before, and if so, when and how, and transfer that information to the user's new profile record for future lead source analysis.

But even then, the dilemma may be: What if they found my site 3 months ago with an organic search, and now they come back through my PPC ad? Which lead source gets credit? I would say that the lead source no more than 5-7 days old should get the credit. If they found some page of your site via organic search, and poked around for a few minutes, or even hours, but never registered, then it gets no credit. Dynamically ISP assigned IP addresses are another can of worms.

Bottom line: Lead source tracking is a difficult and imprecise process. It's not impossible if you only have one PPC lead source, but gets very complex and error prone when multiple sources are in play.

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Ron Goodman, GRI, REALTOR®
Prudential Colorado Real Estate
Denver Colorado Real Estate and Homes for Sale,
AdWords PPC Consulting and Support for REALTORS®
Hobby: Goodman Family History and Genealogy