HI everyone:

I have a Gateway 420GR, XP, Pent 4, 2G RAM, 160GB SATA, Intel 915GV. 

Symptoms:  From a cold boot there is no screen, no access to CMOS.  The CPU fan COMES on and runs and runs very loudly and never stops.  The CD drives have no power but the hard drive does spin and the case fan comes on also.  This happened once before and I switched the hard drive cable from one SATA connector on the motherboard to another connector and it all went back to normal.  That was about a month ago.  There are 4 SATA connectors on the mobo.  None of them work now.

I've taken the RAM out all but one module and switched them around with no response.

I've taken out the PCI network card.

Any thoughts or suggestions would be appreciated.I would return the HDD back to the SATA1 connector...
Then borrow a known good power supply of the same wattage or greater and swap it in there.
You'll know right away if this is the issue...Patio -- Thanks for the quick response.  What I didn't tell you was that in my frustration I had taken the whole system apart in an attempt to try another motherboard -- an attempt which failed.  So following your instructions, I put the system parts back together intending to use a different power supply but didn't have one.  So in a "what have I got to loose move" I simply used the ORIGINAL power supply because it was the only one on hand today and it booted right up.

I agree with your thought to look at the power supply which is probably failing.  I'll go right there next time.  Thanks again.Good News...
And THANX for posting back to let us know...

PS> Do yourself a favor and don't buy one of those 20 Dolllar specials...