Boot problems on emachines 550

[gr19ybryn]

System was running fine till one day last week I came back to find the monitor blank. Tried to reboot - initially came up with XP logo then stopped. Since then nothing. Have checked voltages, changed processor - pentium 4 1.5GHz, mobo - Elite L45SMG and memory. Problem remains the same - fans run, cd light flashes a few times at power on but no communication with the monitor on any error beeps. Have cleared CMOS with the jumper and checked the battery. If it wasn't for the fact that the problem appears the same with two mobos I would have to suspect BIOS but I'm puzzled. Anybody seen this problem or got any ideas?

 

[fishhookz]

Try powering up the barebone out of the case.