Even though VGA mode didn't work it still might make sense to try a known working monitor
It seems your power supply works
Another MB didn't help, it sounds like it shouldn't be a MB problem
Can you boot into setup?
--- If you can, try your XP install disc to see if you can check out some diagnostic utilities
--- If you can't you might need to update the BIOS driver!
--- Do you have a disc with the drivers on it?
--- Who is your BIOS manufacturer?
------ Since this is a home built computer, do you have the MB etc manual(s) for it so you can investigate BIOS beep codes and troubleshooting procedures that apply to this computer?
On the tower, do you have diagnostics lights on it?
--- If so, what colors are the LEDS?
--- It sounds like all of them could be red but if you have them, check them out
On the MB, is there a flicker light?
--- If so what color is it?
Even though the computer was running ok is there something that was happening that suddenly could have caused this problem?
Not knowing how many RAM sticks you have, pull all of them out, install only one back in into slot 0, and try booting into Safe Mode
--- if that doesn't work, work that same stick into each one of the other slots.
--- If that doesn't help, set the tested RAM stick aside and repeat the same pattern for each one of the other RAM sticks
I've read on some sites to disconnect the keyboard. I don't understand how that applies into a no-beep situation but if all else fails, consider trying that idea