My friend's computer is often infect by virus/spyware and sometimes require a reformat. Stuffs that popup to them, they tend to click on it and stuffs like that. How can i prevent such things ?

I decided to install AVG, Zonealarm firewall and Ad-aware to protect his computer but i doubt it will prevent his siblings installing stuffs etc...

What i can think of is creating a limited account for his siblings but i doubt this will help much.. can anyone suggest some tips and tricks to help

Does Limited Account allow to install programs?

Opps... forget to mention WinXp with SP2