Packard Bell - Powers up, but red light stays on

Ok, so it's a Packard Bell UTOW SAN (pentium 4). Around 3 years old now, been left standing for about a year after problem occured, never really had time and then just.. forgot about it.

So everything appears to be connected properly, unplugged it all and plugged it back in.

When i turn the power on, the right light on the power button comes on and stays on. At the same time, my processor fan is on full as is the fan on my card (nvidia Geforce6 512mb pci).

The computer was working fine, until it randomly shut down once. After that, it would power up and boot normally for about 5 minutes, then shut down, until eventually it stopped powering up altogether.

I hope someone can help!

Best guess - power supply. If not an overheating issue - but try another PSU first and see if the machine powers up normally.