Using MSE on XP, one of the smallest. It was available from Windows Advantage website. I don't know whether XP online validation is still functioning, but MSE is still supported on XP.
BTW, Recently had problem with MSE auto updating in the background (similar to Windows 10

). Typically updates come every 30 days. This time virus definition was about 15 days long, MSE reported green, but for some reason decided to get a new built. It started download after 5 minutes with full speed. On Cell C LTE connection it was a minute or two. Update probably failed, downloaded copy was deleted (I couldn't find any temporary files) and after next 10 minutes re-started download again with the same result. This two-shot cycle was repeating each day, consuming 2*38MB=74MB a time. I finally found it was MSE a cause, downloaded virus definition manually from the menu and since then MSE seems to be satisfied. What will happen next months, I don't know. Is any way to disable background udates? I prefer to do it manually. MSE do not use WBT, but some other services.