In desperate need of help!

I was trying to install something on my computer, and it kept telling
me I didn't have administrative rights to do so. So I browsed through
some older posts in this forum and set out to fix things on my own: big
mistake. I right clicked on the C drive and went to Properties. I
clicked the Security tab and then proceeded to go to both Authen. User
and Users, editing them both and giving them both full control. Anyways,
I got paranoid about this (plus it didn't help me in the download
process of what I needed at all), and I went under Authen. User first,
and clicked "deny" under full control. And now? It's gone, the whole
user group is gone!

So I really need help with two things. How do I get that group back?

And what "rights" should be allowed under both Au. User and Users? I
really want to just set things back in order. Please help!