FreeCommander is a very helpful tool to determine what files are essential in XP.
I used it after booting with MiniXP OS 280 MB made with IMG_XP_Compact.exe
Then in FreeCommander when you try to move files from MiniXP to some folder on other drive
then the files that are in use in Mini XP will remain and FreeCommander will do a copy instead of a move.
Such essential files then get a hazy colour so that you know which files to keep.
After doing this procedure for all the files in the system32 folder
you end up with system32 folder containing the essential files. The OS size was then reduced to about 100 MB
I made a filelist by using command window at the root of the drive and giving:
dir /b /on /s /a-d > dir-XPCMD.txt
Later I used
Dependancy Walker to find out which files were missing for a desired process.
Also
Process Monitor turned out to be quite useful.
Open Device Manager and use Process Monitor and use R-mouse to Include Process Name = mmc.exe
Then in column Path all the essential files for that process appear.
For driver update in Device Manager we need according to Process Monitor the following system32 files
that were originally not yet belonging to the build (class installers) but were added in the latest version so that Driver Update is working now

MSIMTF.dll
newdev.dll
batt.dll
bthci.dll
mdminst.dll
mmsys.cpl
sdhcinst.dll
sti_ci.dll
wpd_ci.dll
I hope to get some more feedback about test results.
It will enable me to make further improvements (if necessary

)