Recently I was helping someone move house. Among the appliances to be disposed of in the move were four desktop PCs, running various old versions of Microsoft Windows (from 98 to XP). First I copied the user files from the old computers to a USB hard drive. Then I reformatted the PCs hard disks and used a program to overwrite all sectors of the disk.
I was going to leave the computers in that blank state, with no operating system. But then they would be of little use to anyone. The PCs were so old they were unlikely to be able to run newer version of Microsoft Window, so installed Mint Linux (version 15) "Mate".
For the newest computers I used a USB flash drive to boot (older PCs did not support USB booting and I had to use a DVD-ROM).
The installation went smoothly and the Mate version of Mint Linux provided an interface not that different to an old version of Windows. The installation came with the Libre Office suite and enough applications programs to be usable.