The thing about compresses files most of them are from program or files you created such as picture, documents, even games. They are not the big hogs, as they are compressed down since they have not been opened in a while. So you can delete any old documnets, picturess that you have not used in a while or are no longer needed to you. The same goes with any programs that you installed, if you no longer use them you can remove them. Basicly what I am saying is your compressed files are NOT the issue here.
As for disk cleanup, that is a great tool, but only cleans up the minor things. It wont free up that much space.
In order to free up a large ammount of space, you must remove any program, games that you no longer use. By going into add/remove, but because not to remove any important windows filfe/preogram as that can cause problems.
Now if you have a lot of pictures on your computer, those can eat up a lot of space. So it would be waise to remove those from the computer, but before you do that, either put them on CD/DVD or and external hard dirve (which is a portable hard drive) That is what my sister has done with her picture and that saves a ton of room on her computer.
As for disk defrag, that is not really meant to free up space. It good to use at least once a month, because it does help with the function of the computer. But you do need 15% of free space to use it.
Also if you have any movies or videos on the computer, those can really eat up the hard drive so back those up as well and remove htem from the computer.
If you need more help, let me know. I will need to know the size of you hard drive. The total size of it and not what is left on the computer.