[snip]
> ... 500kHz sampling at PC parallel port...
[snip]

You can start to forget about to run it under Windows (at least for a
while), write a simple timming loop routine (to run on pure DOS 6.22) to
read the parallel port and store the sample data... do it during a fixed
time and display the number of samples done, then you can fine tunning
the time delay so you can have a round number of samples per second.

In an old 20MHz 486 machine you will not be able to sample (and store,
etc) more than 50 to 80 thousand samples per second.  I am telling this
because lots of people use old cheap ($150) notebooks to serve as logic
analyzer and data collector, and most of those run from 20 to 40Mhz...
:)

When you start to migrate this routine to run under Windows, and it
starts to give you problems, and lots of headaches and you want to blame
somebody, just remember Bill Gates.

If you need some parallel port addresses and info, just pick at
http://www.ustr.net/8051pc/8051pc.shtml

have fun,

Wagner.