Lifehacker has a great article on what they consider the top 10 free Windows file utilities. I've used most of these utilities or ones like them. They are all very useful. The WinDirStat is very useful when you suddenly find out that all your free disk space has gone -- in my case, it usually turns out to be podcasts I've downloaded using iTunes.
I recommend that you try out all of these utilities. When you need them, you'll probably need them right away and you will want to have had some previous experience.