I ended up bringing my Macbook Pro into the Apple store. The genius there attached the store's OS X Snow Leopard via USB, and booted from it, and ran the Repair Disk, which gave the same error of an invalid node structure. Since it was still under Apple Care, I got my hard drive replaced and was able to keep the old one.
With the new hard drive, it had Snow Leopard on it, and I simply upgraded it to Lion without a problem. I was able to attach the old hard drive via USB, and restore all my files.
Thanks everyone for their help. +1 to everyone.
In the old days, changing the date, deleting the preferences, or installing the trial software in a different path did the trick, but, nowadays, it seems rather unlikely that there's such an easy way.
Trial software may call home to check the database for a combination of your MAC address and your email, your IP, both, create an invisible file with a who-knows-which-name or a million different things. I mean, if there was an easy way to do so, you wont need the full version, right?
Someone might find a way, of course. But it will probably be for a specific program, not a generic way for all trial software.
EDIT: Just noticed that I'm not answering your question. Indeed, installed and used applications will leave other files, such as Preferences, stored either in your user's preferences folder (~/Library/Preferences) or in the System-Wide preferences folder (/Library/Preferences/), or Application Support Files (which will be, again, in your user's library folder or in the root one). There could be a lot more, of course.
If you want to get rid of them, making a search, with Spotlight or Finder, should show them all.
Best Answer
You can restart while holding down the C key to boot from a valid Mac OS X Install CD / DVD for that G5 based Mac. To pick and choose what disk your machine can boot from hold down the option key while starting the Mac.
Also depending on which version of Mac OS X is installed there are different ways to reset the password in single user mode. This may be an option for you if you just want to unlock and add / remove users and you have a startup CD/DVD for that Mac.
See Changing or resetting an account password for some instructions on how to reset the password.