try recovery console

if you are able to boot to "last known good configuration",try that first.
if not, try this.
change BIOS boot sequence to CD driver first,Boot to XP CD,R for repair,enter recovery console,select 1 if you only have one OS installed,admin password if you have one,then recovery console should prompt c:windows,
then type"fixboot" then confirm that,"[CDdrive letter]:",then enter,"cd i386", "copy ntldr c:" confirm,"copy ntdetect.com" confirm. reboot...
then keep your fingers crossed...good luck

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