I had the same problem with my ThinkPad R40 running on XP. The ADC-42 would function sometimes and not other times. So I installed on a W98SE desktop and got it working about the third try.
Then back to the IBM XP. I tried changing the application and the XP OS to LPT2, then LPT3. No good at all. It showed that the computer could not find the ADC.
And when I changed them both back to LPT1, it still could not find the adc at all. The fix for this is to reinstall the picoscope program.
Now read this: When I had it working "normally", but with that problem that the scope AND the voltmeter sees 5V (most of the time), it made no difference to the performance when I ran picoscope with the adc on the shelf - disconnected from the PC. The program had no problem finding the phantom adc. In fact, now that I fixed everything, it will still operate in phantom mode.
The fix is in the BIOS. Even the bi-directional mode is not good enough for my computer. It needs the ECP mode. Then it works every time. In bi-directional it works some of the time. I cannot find out why.
I got a hint of the fix when I googled the issue and found a similar problem when you try to connect two computers via parallel ports using Symantec Ghost. (I think Norton's original Ghost - the good simple one - was exorcised.) They liked EPP but I used ECP.
Are parallel ports so obsolete that MS does not support them? Am I going to be able to connect anything other than a toaster to the next version of Windows?
As for Pico Tech, you guys need to give warnings about these port incompatibilities. Signals that go from the port to your program need to navigate treacherous seas. So tell us.