PC Cycles... But Doesn't Boot

From jagxk in the PC Support Forum:

Updated Quicktime and the final step was to restart the PC (running XP Prof). The screen which follows a crash and offers: normal, safe mode, etc appeared. Clicking any of the options produced the same result, a brief flicker of the Windows logo before the screen cycled through a power saving message, the manufacturer logo and back to the first screen with its start options.

I have tried starting using the original product recovery CD-ROM from F-Siemens but even clicking random keys during the start-up process produces no result other than freezing the 30 second countdown to "start normally" as does clicking F2.  A 'wiggle' test has not made any difference. The PC has no previous history of this sort.

How do I begin to analyse and then correct the problem when I can't get it to turn on?

Any thoughts for jagxk? Head over to the post and see if you can help.