OK, I gave up on using that computer because of the various problems encountered. The original problem must have been due to the printer port, which on that computer can only be set to EPP/ECP/Bidirectional, none of which are suitable for adc100. And the USB interface is just too annoying to perservere with.
So I dusted off an old Dell workstation. This has a good LPT1 port that can be setup correctly to interface to the ADC100.
Dell runs win2K; printer port adapter is used. [ADC100]UseAdapter=yes is in win.ini
Here's what happens, using R.20 software/drivers
Picoscope - no problems initially. Gives good readings.
VB example code - usually works, but sometimes complains that it can't find pico.sys.
C code - (this dynamically loads the adc100 dll, so it always uses the latest version) sometimes complains that it can't find pico.sys; however, depending on where this C code is called from within Matlab, sometimes pico.sys is never found. In addition, the code, which previously reported scaled voltages, now reports unscaled integers (now that's a bizarre change, since the C code has not been recompiled)
Once pico.sys is lost, it stays lost for a while. Usually restarting picoscope is enough to refind the lost pico.sys file, but not always.
I noticed a similar problem on another forum post, where pico.sys wasn't found, and you suggested using the latest software. This I have.