Completely reinstall os

I have a laptop I’m going to sell on ebay. I need to completely clean it up and have nothing on there but Windows 98 OS, which it came with (yes it’s old).

I installed the boot disk and the OS CD and started the computer. After going through the process and following all the instructions (took about 30 min) is started up with everything still on there! :upset:

Is there something I missed? I was always under the impression that starting up from the boot disk would erase everything and reinstall the OS on top.

thanks for any help