Perhaps the execution angle would be simpler. The joke is taking the idea of annoying banner ads and pretending they are real, the idea being no one will know they are real. OK. But the problem is this: The punchline should be concise. The idea of "a genuine banner ad" should hit you all at once, preferably in the final panel. Because that's the only source of humor (which is fine, most comics have one joke) it should be made as powerful as possible. But it's not here: The second panel makes you think of banner ads, the third panel tells you no one clicked on it, and the fourth panel...well the fourth panel just says the ad was flashing, which is either implied in "annoying banner ad" or irrelevant.
This comic could stand to be condensed to 3 or even 2 panels. "What's the car for?" "We bought it for the Xth visitor to the site but apparently he missed the notice telling him he'd won." Done. The end. As it is, the last panel could be made into the alt-text and it would work better.
Of course, the comic would still suck. Because the idea of scams being real is old. The Dreaded Forums were brimming with examples, such as this Least I Could Do or this skit from Mitchell and Webb, which just so everyone knows, is broadcast in america. My own thoughts immediately went to the Leon Sumbitches Achewood story, which is 8 strips long so make sure to keep clicking "next." All are executed differently, but there is nothing in the xkcd that isn't present in all of those as well.
I know Randy reads achewood. He has it linked from his page, and I'd hate to think that he links to comics he doesn't read just because other people like them. So I'm not going to say randy is copying other people's ideas (again), just that these ideas are pretty common and xkcd is taking the lowest of the low hanging comedy fruit. But if you think he copied the joke...well, I won't stop you.
late update: IT'S A GOOD THING I'M GOING THROUGH THE OVERCOMPENSATING ARCHIVES! because I just found this, which entirely has the joke of xkcd's alt-text contained within it. But with rhymes, colors, and recurring characters. On an unrelated note: you should read the overcompensating archives.