There are other files that are not addressed by the Disk Cleanup utility.  There are gobs of temporary files generated by the operating system and your applications that just hang around after they are no longer needed, and they are not always in a Temp folder  The number of these files will vary greatly from one user to the next, depending on installed applications and the normal daily use of the computer.  These files end in .tmp and are generally no longer of any use once the session that generated them is closed.  A lot of these files reside in the Windows folder or Windows subfolders.

These files are most easily cleaned out using Windows Search.  Go to Start > Search  (If you have installed the Windows Desktop Search, you'll have to scroll down to Click here to use Search Companion, and click there) and then click on All files and folders.  After you have opened the dropdown for All files and folders, in the textbox for All or part of the file name: type in "*.tmp" (without the quotes).  Then click the dropdown for More advanced options and put a check in the box by Search system folders, Search hidden files and folders, and Search subfolders.  Then click Search.

When the search has completed, click on Date Modified at the top right of the pane displaying the list of files to sort them by oldest first, and then click Date Modified once more to resort them into youngest first.  There will probably be a handful of folders such as msinst.tmp or msdownld.tmp or an assortment of letters and numbers.tmp.  These are usually empty or contain only one or two small files, and can be ignored.  I usually delete everything that is older than two weeks.  If you wish to be a bit more conservative, go for everything older than a month.  The easiest way to delete a large number of files like this is to scroll to the bottom of the pane (if you have sorted for youngest first), and left-click the last file.  Scroll back up to the youngest file you want to include (two weeks old, one month old, or whatever you feel comfortable with) then hold down the shift key and left-click on the file.  This will highlight all the files in between.  Release the shift key, then right-click in the highlighted area and select Delete.

Leave these in the recycle bin for three or four days.  If your computer continues to operate normally and you don't get any messages from Windows about unable to find... files, then you can empty the recycle bin.  I recently recovered 3.5GB of space on one of my PC's that I hadn't run this procedure on for quite a while.

To clear out what's left

there are just a few more steps. There are temporary files generated and placed in all sorts of places in addition to the \Temp folders.