How can I fix my iMac's permissions?

Before you say “use repair permissions in disk utility”, I’ve already done that, and it hasn’t helped my current problem.

Back in the days when i was still a mac noob, I was (stupidly) playing around with permissions in several different user folders, thinking at the time I could make the file sharing easier between the several user accounts we have on the computer.
However, it has made things difficult in certain areas. For example, my sister can’t edit her desktop without authenticating as an administrator (even though she has an admin account). This is just one example of some of the problems I’ve been having.

Some of the folders I can fix up when needed, but I don’t know what the defaults are. I don’t want to go making things easier while compromising security at the same time.

Is there any way that I can get the user folders (such as the user root folders, desktop folders, etc.) to reset back to their defaults, without doing a full reinstall of OSX? Anyone know of any software I can use for that?

I’m going to use a full reinstall as a last resort, but my family has a lot to back up…

From what I can tell, “repair disk permissions” doesn’t effect user folders, only system folders.