|(Specs and make/model is of course alway useful when posting here...?)|
Power down; remove power into system completely; and if a laptop then remove battery too.
If a laptop, remove RAM sticks; clean connectors (soft pencil eraser is often advised here); re-install RAM; reinstall battery and mains power etc. See what happens when try to reboot again...
If a desktop/tower then as above remove all power - no connection to mains at all.
Open case and check all connections to motherboard are secure; clean RAM as above; repower and verify all fans (cpu and psu) are running correctly; and see it will boot up etc. OK?
In either of the above... if you uhave two (or more...) RAM sticks installed - reduce to a single (min of 128Meg) and try each in turn.
Do you have a full version (or upgrade) XP CD (or access to one)?