Restoring my computer to factory and keeping apps

I have a power G5 dual processor with mac os x 10.5.8 leopard installed.

I need to erase it to how it was when I got it. I do not have any disks, since I bought it refurbed from a local guy.

I need to delete my admin account (yes I do have password) and create a new admin account

I dont want to lose any applications or software on the comp, such as the iMovie iDVD etc... but I want to get rid of anything I downloaded from the internet as well as my personal files I have saved. It seems like a lot of my stuff is all scattered around.

How can I restore it back to original clean install, keep my apps and software, but delete my personal files and downloads?? I also have my 500 GB hard drive split into 2 partitions, and an airport card installed etc..

Thanks!