To get OXCE work on my win 2000, I just butchered the exe as well as several DLLs into submission.
OXCE checks for several DLLs in Windows/system32. While it is unhealthy to just copy DLLs from WinXP SP3 to Win2000, you can hex edit the exe to look for the DLLs in the wrong spot, or for a wrong filename (easiest). A fallback is then executed to look for the missing DLLs in the installation folder.
Then download the DLLs that are looked for in their WinXP SP3 version and put them into installation folder.
Next you have to hex edit those DLLs inside the installation folder to prevent themselves to look for other DLLs in Windows/system32. A wrong name works well here, too. Again a fallback is executed and the DLLs from the installation folder are used.
Once the DLLs are setup properly, one has only to edit the exe in case it is replaced.
This procedure completely bypasses the installed windows version.
It is possible this would work even down to Win95, but may require additional DLLs to be butchered. I'll test that in a quiet minute on one of my really old machines.
The new guide is great. This makes things a whole lot easier especially for people who have zero experience with MinGW compilers. Well done.
EDIT
There are a lot of differences to the exe I got from Meridian/the Piratez mod.
My exe is 17 MB instead of 11 MB, does check for additional DLLs, and does not have the alien head icon, but dos prompt icon.
I used OXCE 3.5 plus proto code from here:
https://github.com/MeridianOXC/OpenXcom/tree/oxce3.5-plus-protoDoes this difference result from different code, the compiler itself, or the makefile?