How to set user privaleges on an administator account?

My dad has my computer set up with an admin account so that i don't give it viruses with downloads, although he has given me the password, defeating the purpose. Unfortunately, this means it is really complicated to download things. I can only download on the admin account, but then whatever i download there can't be used on the other accounts, which is where i want to use them. My dad has fixed this problem on an individual basis, but he forgot how, and i need to know how. I downloaded team fortress 2, and it only appears on the admin account. How do i set the privaleges so that i can play it on the regulars account? I think i have windows xp