Running out of hard drive space problems

I am getting an error message indicating that I am running out of hard drive space. I've tried to delete "snapshot" files, but I then get another error message saying I don't have enough space to do that. Since I use snapshots, I can't use the image compression program or the compress program. My 50 gb windows VM is now at 178 gb on my OS X hard drive. How do I reduce the sizes of my files? I've done a disk clean up on the Windows Vista VM side of the equation. Can I do a OS X (10.6.4) compression of the Parallels 4 hdd file? I'm at a loss on what my options are.
Thanks, Dennis