I just can't find a way to monitor the percentage of space used
on a logical disk.

I access W2k performance monitor via pdh.dll and this brings up a
dialog where you can select individual performance-counters to
monitor. It is the same dialog that appears if you start the system
monitor:

start/programs/administration/system-monitor and select counter-add
there.
(sorry if this does not match exactly to your system. I had to guess a
bit because my system is not in english language)

I find an object physical disk, but none of the indicators seem
to give any information about the size of a logical disk or of
the space currently used on the disk.

Is it possible that such information is not provided by w2k?
Or can anybody give a hint how to find the info?

TIA
Harald