I had this happen on a system. Not an XPS, but an Optiplex. Turned out it was the CPU in the end. Not before I replaced the motherboard 2X and RAM 2X because I kept getting contradictory diagnostic codes.
However, I have also had systems (I fix Dell systems for a living) where a solid amber power LED meant the motherboard went bad.
In either case, unless the system is under warranty, it's probably not worth the expense of fixing. And the odds of a P4 system still being under warranty seems pretty slim.
You can troll ebay listings looking for parts, and there's also a place I use when I need to do an out of warranty repair called www.used-pcs.com. Just call them up, tell them what you need, and they'll tell you if they have any, what it costs, etc.
I recently inherited a Dell XPS Gen 4. Pentium 4.
The computer was "running slow" from account of previous owner, hard drives were around 99% full. Computer was working fine, no crashes,just slow. Then it was transported two states via car to me.
On arrival it would not start.
Turned it on and got blinking amber power light, which according to Dell means power failure,so I replaced the power supply.
Same thing blinking amber power light, with a green light on on the mother board. So I unplugged EVERYTHING and item by item put everything back in testing it each time.
PC will power on, not boot, but power on with a steady amber power light if the P2 12v CPU power is not plugged in. If I plug it in it is dead as a rock and I am back to the blinking amber light. If I plug in the data cables for the hard drives, DVD drive and floppy, then I get a steady amber light bit they will not try to start up. If data cables are unplugged then they try to start up and I still have a steady amber light.
I am trying to figure out if this is my CPU or the mother board. Any advice would be great.