Copy files automatically

I would like to have a set of folders that mirror identically another set - a backup of sorts.

I also only need the copy to happen once per day.  I don't want to take the time or resources to copy all the files each time as there are around 100 gig of files but only about 4-5 gig change per day.

Is there a free program, a program native to windows (2003 or xp or 7), or a fairly inexpensive program that will accomplish this?

As an aside, the windows backup program saves the data in one large file...I would prefer to just be able to navigate to the folder as if it were the "actual" original folder if I needed something.

Thanks...