My grandfather is pretty tech-savy for his age and loves to send emails, use Photoshop, and browse the web. I've made his computer pretty secure by installing AdBlock and teaching him how not to download viruses but every once in a while he receives a flyer in the mail with a free trial for 'Avast antivirus'. He's the type of person who can't resist free and promptly installs it. This is pretty annoying because it slows down the computer a lot, prevents Firefox from opening, and does all sorts of other this-antivius-is-basically-a-virus things. It's also a real pain to uninstall.
My question: Can I set something up that prevents him from downloading, installing, or running an Avast installer?
I have Windows 7