You could get an installer DVD (maybe). You don't have to look for a proprietary DVD, just the normal system installer.
Tiger might be a problem, as you actually would need a specific DVD. The intel version is not easy to find. Newer systems, Leopard, Snow Leopard, or Lion, any of those would be less challenging. I would go for Lion (Lion can always be on a USB flash drive, and you can make your own for that. The system installer can be downloaded from Apple (for free), and you just follow a couple of simple steps to make a USB installer. Boot to that, then erase the drive, and install Lion on the erased drive. Then you will know what you have. If you try to just remove the existing user, you still don't know if the system is correctly installed. Starting from a wiped drive is much better, IMHO.
Finally, you might find that your nearly-twenty-year old optical drive may or may not work to install a system.
USB bootable installer might be more likely to actually install a system.