Uh Paul, neither of these programs blocks ads, well I don't know about spybot, and if you want ad-aware to block ads you have to pay for it, but that's besides the point. Ad-Aware is used to delete many types of software that is installed on your system without your knowing about it. It can cause a lot of problems just by being there, taking up PC space.
1. It can cause a slower boot up on your pc.
2. It can slow down your computer.
3. It can mess with your homepage, sending you to god knows what web page when you start up.
4. It can toss a tail load of new webpages under your favorites category.
5. It can take up your bandwidth reporting information about you back to the person who planted it on you.
Now I know all that sounds fun, but it can also start chains of pop ups that don't end. They don't end. They won't end. Annoying isn't it? Sometimes the only way to stop them is to pull the plug. I've had to do that a number of times. And of course these pop ups aren't advertising cheetos, red bull, or nike, noooo, their advertising the 1# industry on the internet. And that's what 99.99% of the ads popping up are going to be.
There's a bunch more, but there's plenty of people out there who know more about spyware, and you can go to their websites to read all about the fun stuff spyware does. So just download a copy of Spybot or Ad-Aware, and kill off all these little bugs before you start to get bit.
For Spybot
http://spybot.safer-networking.de/index.php?lang=en&page=download
For Ad-Aware
http://www.lavasoftusa.com/support/download/
For more info about spyware
http://www.spywareinfo.com/
And for the record, I like Ad-Aware 6.0 It's interesting to watch the program scan, and read what baddies you have on your system before you delete them. That and it's easy to update.