PiCTeX is a resource hog; fortunately, most modern TeX implementations offer generous amounts of space, and most modern computers are pretty fast, so users aren't too badly affected by its performance.
However, PiCTeX has the further unfortunate tendency to fill up TeX's fixed-size arrays - notably the array of 256 'dimension' registers. This is a particular problem when you're using pictex.sty with LaTeX and some other packages that also need dimension registers. When this happens, you will see the TeX error message:
There is nothing that can directly be done about this error: you can't extend the number of available! No room for a new \dimen.
\dimen registers without
extending TeX itself.
e-TeX and Omega both do this, as
does MicroPress Inc's VTeX.
It's actually quite practical (with most modern distributions) to use e-TeX's extended register set: use package etex (which comes with e-TeX distributions) and the allocation mechanism is altered to cope with the larger register set: PiCTeX will now load.
If you're in some situation where you can't use e-TeX, you need to change PiCTeX; unfortunately PiCTeX's author is no longer active in the TeX world, so one must resort to patching. There are two solutions available.
The ConTeXt module m-pictex.tex (for Plain TeX and
variants) or the corresponding LaTeX m-pictex package provide
an ingenious solution to the problem based on hacking the code of
\newdimen itself.
Alternatively, Andreas Schell's pictexwd and related
packages replace PiCTeX with a version that uses 33 fewer
\dimen registers; so use pictexwd in place of
pictex (either as a LaTeX package, or as a file to read
into Plain TeX).
And how does one use PiCTeX anyway, given that the manual is so hard to come by? Fortunately for MSDOS and Windows users, the MathsPic system may be used to translate a somewhat different language into PiCTeX commands; and the MathsPic manual is free (and part of the distribution). MathsPic is written in Basic; a version written in Perl is under development, and should be available soon.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=usepictex