Self-built system up and working using 98 no problems, then l got too clever and decided to upgrade to XP.
Formatted hard drives, installed XP, and the screen quaintly counted down "your computer will reboot in 5, 4, 3, 2, 1", and then didn't bother.
The HD spins, the lights all flash and wink, and the monitor does nothing.
The power indicator on the monitor initially goes green, and then straight to a cute red flashing light. No signs of life from the monitor at all. I can't even reinstall 98, due to snapping the CD in half.
Any body got a clue how to dig me out of the brown stuff??

Does that happen every time? Because if that happened once, then your computer won't turn on at all, you may (I've seen this once before at my neighbor's) have a blown power supply. Let me know!

It happens everytime. Green light on the monitor for a split second, then flashing red/amber. Other than that, there is no activity from the monitor. The base unit powers up, and the HD spins, but apart from that, its a very large paperweight!

I would put a win98 boot disk in run fdisk from prompt. Partition you drive making sure main drive for xp is 15 gigs.

Then install xp again. Myself I wonder if your computer fits the specifications (minimum) required to run xp.

You haven't listed your specs, so that's all i can suggest, fdisk, partition how you want install xp and start again.