Security Pipeline have an article that looks at how sites that are aimed at children are particuarly infested with Adware.
From their findings they’ve found that children’s sites particularly have Adware, rather than spyware – which, they assume is because children don’t tend to have credit cards, so it’s not worth spying on what they are doing.
These were machines that were new PCs, with XP SP2 set at the default settings and they found:
Children were the biggest target for spyware makers, by far. The trip to several kids’ sites installed a whopping 359 pieces of adware on Symantec’s PCs, five times more than the nearest category rival, travel. Popup ads proliferated on the machines after that, making them virtually unusable.