Do I need to buy Mac OS X again? I am running 10.6.8 on my Mac Pro 1,1 and I want to reset it. I don't have the install disk anymore, so do I need to buy it again? Or can I copy the files from my system, and put them on a disk and boot from that? (I would prefer not to reinstall the OS, I just want to delete files, as it is getting pretty full, is there a way to only do that without completely reinstalling the OS?)
I would say the best option might be to get software like Carbon Copy Cloner or use Disk Utility to clone the existing drive to another drive / external drive.

Then you could clean off the files on that OS and test that booting to it still works. Once you have something that works - you can re-clone or repeat the deletions on the main drive.