Spooler was always set on.
However, there is not much installed on this computer, which is devoted to running one programme written in VisualBasic 6.
To try another virtual printer I had to install .net Framework 4.0 first. I installed .net 4 but decided for fun to try out pdf24 again before installing the other virtual printer. Maybe it is unrelated, or maybe it is, but now pdf24 seems to work fine!