im not sure if its still possible, since most of the guides have died, and are not being able to be retrieved from the logs, {just like the old links, they have either been removed or are not accessible anymore}.
but if still have the files you could try to install it like any mac, {if you want dual boot i would suggest to start with the other OS first}
but i would not suggest it since it might run very very slow.
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i also still use my dell mini from time to time, but not soo much anymore.
im planning on stripping it down, {removing the screen so its energy consumption is lower} and then try turning it in a smart home control base.
using raspberry pi zero and esp8266 devices