Some graphics card drivers seem to be just screwed with Disgaea 1 and 2 on PC. If you update your drivers or rollback to an older version it might work. I have the same issue and luckily can get around it since I can play it on my laptop, just not my desktop.

A quick addendum: The Pretty Prinny mod might be able to fix the issue. I tried and through some fiddling around with the compatibility settings in the .ini files, I was able to get an older version of the pretty prinny mod to run Disgaea 1 PC somewhat fine.
There was some issue with the shaders so everything was rendered extremely dark so I gave up at that point.

Developer already has our money and doesn't care. I'm not engaging in a 10 minute PC reconfiguration just to play one stupid game.