First, you could simply add another Administrator account under your name and use that instead of the pre-installed user account. (You can actually delete it after setting up yours.) Just make sure that you have backed up any data files you might still need, so you can access those files from within your _new_ user account.
Other than that, you can, of course, re-install Mac OS X completely using the original installation disks that came with the computer. (Thy did, didn't they?) This way, you'll end up with a clean system where you decide what's installed, what's not etc. However: By doing that, you'll lose any information that previously was on the Mac.