HELP!! How can I remove all previous owners files, docs, etc without restoring

I bought a used late 2009 MacBook & Mac Mini HTPC loaded with apps/programs and this is my first run with Macs. The previous owner didn't restore them at my request as part of the deal, all of the software discs and serial numbers went with his other MacBook when he sold it and if I restore I will lose everything but iwork and snow leopard, I can't afford to buy new and don't want illegitimate copies of CS5 Master collection and tons of others that can't be updated. I have spent two days attempting to remove his what seem to be thousands of files, docs, etc and seem to be getting nowhere and don't want to accidentally remove any operating components either. Is it possible to delete the user that installed all this software to wipe out his info/files without losing the software itself? He is the only user besides "guests", "staff" and "everyone else", I renamed it once but it seems to revert when I go to another app??? HELP the Newbie please.

Thanks in advance