Microsoft is getting better when it comes to default lockdown modes (Vista) but I believe Administrator by default is the reason why Conficker et al and botnets exist.

I can't think of any good reason why the average home user needs to run as Administrative user, other than convenience.

I have to "administer" my dad's old computer and I gave him a Limited User account. Yes, he complains about not being to install some software his friends email but no viruses, no spyware. Cuts down on support calls and unnecessary trips home.