Your Mac needs adequate hard drive space to operate normally. How full can a drive be before it's too full? There is no hard and fast rule that says “X” amount or “%” of free drive space is needed. A low amount of RAM requires more drive space for Virtual Memory’s swap files.
Problems from insufficient RAM and free hard disk space are discussed in this link
http://www.thexlab.com/faqs/lackofram.html
Here’s some general guidelines for minimum hard drive free space:
1. As a general rule, your available space should be 5GB as an absolute minimum as it generally requires that much free space to perform an Archive and Install of Mac OS X and still preserve some free space for VM swap files.
2. Some say that your hard drive should have at least 5% of it's capacity available for use. Still others say 10% to 15%. If you routinely process complex graphics and videos, even more space is required.
With your 19.7GB of free space, you should be OK. Usually it's best to not state a percentage, but state an amount of free space. There's a big difference in having 10% free space (20GB) on a 200GB hard drive compared 10% (4GB) to a 40GB hard drive.
Look at these links about freeing up more space.
Where did my disk space go?
http://www.macfixitforums.com/showflat.php?Cat=&Board=Forum38&Number=770243
Download & use WhatSize described in this link or Disk Inventory X @ http://www.derlien.com/
Freeing space on your Mac OS X startup disk
http://www.thexlab.com/faqs/freeingspace.html
Amazing Disappearing Drive Space
http://www.pinkmutant.com/articles/TigerMisc.html
Increase HD Free Space
http://macosx.com/forums/howto-faqs/275191-how-easily-increase-hd-free-space-lap top.html
How to free up my disk space
http://www.macmaps.com/diskfull.html
 Cheers, Tom 😉

Hello All,
I'm having a pretty serious problem. I attempted to erase the free space on my HD, as I do from time to time, except this time it appeared to hang/freeze at the end, when it gets to the part where it creates a temporary file. I waited for several minutes and it didn't move. I tried clicking the skip button, but that did nothing either, so I force quit Disk Utility.
The problem is, it left the disk at 'Zero KB' of free space, effectively making my computer inoperable. I was in the process of studying for a huge exam tomorrow, and am currently flipping out. Any help would be greatly appreciated.
If it matters, I'm on a Macbook Pro 2.33 GHz Core 2 Duo, running 10.4.
I really don't want to have to reinstall the OS and import the old files, but my real fear is that I will have to do a fresh install of the OS.
Thanks in advance,
steiney