foolproof computer from malware?

One of my neighbor has managed to acquire all sorts of spywares and maybe
even a rootkit.

I'm going to reformat and re-install windows on his computer.

If I make his account a limited user account, would that stop all malware
from entering his computer? How else can I make his computer more foolproof?

Is there any browser proxy that would *automatically* block all the bad
popups, activeX, and other bad downloads? It has to be automatic. If the
user has to answer a question "do you want to install a virus or a rootkit",
my neighbor may click yes by accident.


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