Good idea. This is what I did with my ancient Dell laptop. I bought a new disk from Frys, a new drive caddy from ebay, and installed Linux on it. I still have the old Win2K disk laying around somewhere.
I’ve got a menagerie of laptops in addition to a couple desktops, and numerous old drives laying around, some blank and some with working OS’s and apps. I’ve found it a good idea to mark on each HDD what OS is on it.