It's a scavenged computer, so we can assume that there is nothing that you may want to keep.
Just pop in any operating system install disk (it can be Windows or any Linux distro) and do a clean install: Format the drive and install the OS.
You'll get a partitions manages step. In it, suppress any existing partitions.
Create the required primary partition.
Optionaly, create a second partition for the data.
Electro, August 2012