It's unclear if its broken or not. Some machines do that on shutdown and a review of the XP Shutdown Article two clicks from http://www.aumha.org/articles.htm would be helpful. But if it never boots, then disregard that and tell if its broken.
If under warranty, call it in.
If not, cry a little and try this. Unplug and remove any part you can. Reassemble and try again...
I have a Compaq Presario 2100 series with Windows XP installed on it.. i'm not sure if this is the right place to post this but i'll ask it anyway;
My computer will power on for about 3 seconds and then power back off.. and then repeat this pattern until you take out the power cord from the back..
Any ideas on what could be wrong?