Written 23 Aug 2026. This is a how-to, not a product card. Buy links are not tagged yet.
What OS to put on the leftover box
Debian or Ubuntu Server if the box is dedicated. Leave Windows if you still use the laptop as a laptop. Proxmox and NAS operating systems wait until you have a reason.
An OS is the software the computer boots into. Windows, macOS, Debian, Ubuntu. You pick one, install it, and that is what the leftover box is from then on. You do not need to buy a new computer to pick an OS.
If it is still your daily laptop, leave Windows, or macOS if that is what it has. You can learn later. Do not wipe a machine you still write papers on.
If the box is leftover and dedicated, an old laptop in the closet, or a Marketplace OptiPlex, install Debian or Ubuntu Server. Both are boring, free, and every guide on the internet assumes one of these two.
Ubuntu Desktop if you want a screen and a mouse while you learn. Same family as Ubuntu Server, with a picture of the desktop instead of only a text login.
Debian if you want fewer surprise changes. Slightly less hand-holding than Ubuntu. Same idea: a free Linux you leave on.
Proxmox is for when you want several pretend computers, called virtual machines, on one box. Not day one unless you already know you want that.
TrueNAS, Unraid, and OpenMediaVault are file-box operating systems. Later, when you have disks to fill. See a first file box when that is the job.
What we would do
| What you have | What to put on it | Spend |
|---|---|---|
| Daily laptop you still use | Leave the OS it has | Never for this |
| Dedicated leftover box | Ubuntu Server or Debian | $0, a USB stick you already have |
| You already know you want many VMs | Proxmox | Still $0 |
| You already have disks to fill | A NAS OS, later. See a first file box. | Not this weekend |
Skip
Skip paid hypervisors, Windows Server, wiping a daily driver, and buying an N100 just to run Linux. Kubernetes can wait. The leftover box already has a disk. Use that.
Sources
Debian download. Ubuntu Server download. Proxmox downloads. Official pages only. For the leftover box itself, see start with what you already have. For the install, see your first weekend.