Hard drive space missing.

Do you have this problem in your Windows XP computer?
My Computer shows: Total Hard Drive Size = 60 GB; Free Space = 11.6GB

1. Clicking on Properties of ALL files on C: (include hidden, system
   files etc.) yields = 15.9GB

Solution:

1. Clean up your disk using the disk cleanup utility. Select all to
   clean.
2. Make sure that you considered the hidden files and folders when you
   try to find out the total size.
3. Check the drive with anti virus.
   Still you have the space difference, try the following.

For temporary purpose do the following and find out and change in free
space.

1. Turn of your hibernate option in system power properties. (It takes
   almost the size of your RAM)
2. Reduced the allocated size of system restore to minimum in system
   properties.
3. If your drive is partitioned, change the page file size (virtual
   memory) to a different drive and after all restart the system and
   check the file size.

If you see the change, do not forget to make all these changes to
previous condition for your system stability.
Note: If the space missing in a different drive, run Drive:\recycler.
Its an recycle bin for each drive. Clean it.