It's not clear to me which version of Doom you're using, since HPC Doom and Doom4CE aren't quite the same version. HPC Doom is from the Gamefreaks site at freeuk.com, while Doom4CE is from the now-defunct Jimmy Software
(still accessible through the Wayback Machine at archive.org
). Which one do you have? Also, have you also downloaded a doom.wad file onto the 790?
(Doom4CE also needs Doom4CE.wad - also from Jimmy Software - as well as gx.dll.
) All the files should be in the same folder, and it runs more reliably when installed in RAM rather than on a Storage Card.
(Have I succeeded in confusing you further?
)
mscdex, Doom4CE does have MIPS and SH3 versions as well, but they were written for the Palm-size PC rather than the H/PC. However, it's possible that they'll work on the H/PC.