get to show the Press DEL to enter setup [or the equivalewnt]??
Does it get a beep [or more]??
An unusual power supply problem could cause this. There is a signal that the power supply [called power GO or OK] "releases" from 100 to 300 milliseconds after the supply turns on. Before that time period elapses the Power GO line holds the master reset line "down" and then releases it after the time delay. The delay is to give time for the power supply voltages to get up from zero to their regulated value [it can't happen in zero time]. Release of the master reset line sets the program counter to execute the first steps of the boot process.
That signal is on Pin 8 of the power connector to the mobo. I'm not sure of the signal levels, but if you have a voltmeter you wight want to see if it keeps changing levels.
I just got an Antec supply replaced because the voltages all came up fine but the computer wouldn't boot. Thought sure it was a bad mobo, but tried the supply on another mobo and same problem. Different supply worked fine. In this case the Power GO never changed state.
Some good supplies actually check the voltage rather than a simple timeout.
THUS, it is possible that something in the computer is overloading the power supply and the supply then shuts the power go signal off. So disconnect some of the items such as hard drive and floppy [power and signals].
I am helping a friend out and her computer continually reboots before BIOS kicks in. Help!