You can compress up to 50%, or better, with text, documents, data files,
etc. I would not recommend compressing your entire hard drive, as it will
impact your performance!
Selectively compress folders containing files, as noted above. A good place
to start would be your, My Documents folder. Files like *.zip, *.rar,
*.jpg, *.mov, *.mpg, *.avi, are already compressed, to re-compressing them
is redundant, and will produce little, if any results.
Other tips to regain drive space are; delete *.tmp files, cookies, temporary
internet files, applications no longer used, or needed. You can also reduce
the size of your recycling bin. Windows defaults to using 10% of your hard
drive for the recycling bin, which on today's larger hard drives is
overkill!
When you're done with all of the above, defrag your hard drive.
Bill Crocker