|There could be any number of component failures preventing your system from booting. First step is to re-seat all cards & cables to see if it isn't just a loose connection somewhere.|
Next possibility is faulty powersupply. Without a known good powersupply you cannot test further.
After that you need to strip the system down to barebones. IE: Disconnect all drive power & data cables, remove all but 1 stick of ram, remove any/all PCI cards, Connect to onboard graphics and remove any AGP graphics card if onboard is available. Leave only powersupply, motherboard with CPU Heatsink& fan, 1 stick ram, keyboard & graphics. Now connect power and see if you can boot up into bios. If so then add back one component at a time until failure occurs.
If no barebones boot try removing all ram. Do you get any bios beeps? If yes then CPU at least is ok and motherboard functions. If no bios beeps at all then your problem is either failed CPU or motherboard. Given you are running XP on a system with only 256mb ram it is obviously an older unit maybe P3 or P4 or similar vintage AMD. Not really worth spending much on replacement parts.
Goin' Fishin' (Some day)