After years of being perfectly well behaved my pc has, just this morning started restarting itself a few seconds into the startup. It gets to the very first screen and when it shows "Initialising usb controllers" the monitor goes to stand by, the lights all go off then it starts again. It seemed not to vary or get to different points but I didnt just let it keep doing it as I was totaly unsure if it might be a power problem, a hard drive problem, a problem with some random component or with a usb device or port. I unplugged the usb devices and it seems to make no difference.
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Is it during the it initialising, which u put or does it get to loading windows Xp and wat is the message that comes up or is it just restarting continuously.?

Sounds as if there is either a software corruption of some system files or maybe your HDD is failing to read a certain sector which contains the system files needed to boot into windows.

First of all, try to get into bios to see if the display stays on. And then see if you can freeze on BSOD - thats if you are getting one.

Secondly, i would suggest using BartPE to replace hive files and do a system restore. Or to just simply re-install windows onto a new HDD to rule out the fact its anything to do with your computer.

If you do a google search you will find there are lots of bits of software for hardware testing.

Start it in safe mode. Click Start, then Run. Type msconfig. Click the Startup tab.
Google each item on that list if you don't know what it is. Disable it if it's malicious (causing restart, or other virus), or if you just don't want it to run by default (as it slows startup speeds).

I don't recall Windows XP ever saying "initializing USB controllers" during startup, so I assume this is post-login, and safe mode is still accessible.