Thanks a bunch mate, will try out immediately and report back!
edit: Works now, excellent. I tried to make it into a vertical layout (by which I mean make it into a vertical layout that works with a horizontal windows desktop orientation - see below for why I can't rotate windows itself), which is what I need for my cab, by inserting "Vertical Orientation" in the section
// Layout Constants
...
if...
else
{
// Vertical Orientation
Nothing changed though, I still get horizontal orientation. (It's obvious I operate by trial and error, sorry if the solution seems obvious) If it's not too much to ask, do you think you could make it into a vertical layout in the above sense? I use a 4:3 LCD monitor at 1024x768. No worries if you won't, of course...
One more thing: apparently, vanilla MAME works well with a rotated desktop, unfortunately I use GroovyMAME. Calamity, who is (one of the) programmers on it, says in an old BYOAC post that GroovyMAME doesn't work with a rotated Windows desktop. I'm using the most recent version (0.220), and it still doesn't seem to work (4:3 games are stretched to full screen in MAME (in 3:4!), and the rotate option in the TAB-settings doesn't help.