There are safe and not safe programs... I would say that Zangocash and Vommbacash is safer than others.
Usualy you find this when you download something free from the Internet and you dont even know about it.
Than you get popup messages and your homepage is changed. So the company get few clicks and few sales and you get your share.
That is why they can pay you.

So if you have site with higher traffic than you can put there ActiveX and than if someone wants to access your content he has to install and you get paid.

Or if you have some software for download you bundle it and submit it everywhere or leave it on your site and make money with every download... up to $1.20 per install.