This works for Windows 2000 when it won't boot, and I assume will work for XP as well.

From the recovery console (accessed by booting from the XP CD), type these commands in succession:

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Set allowRemovableMedia = True
Set NocopyPrompt = True
Fixboot c:
FixMbr
Chkdsk c:
Attrib -r c:\ntldr
Attrib -r c:\ntdetect.com
Copy d:\i386\ntldr c:\ntldr
Copy d:\i386\ntdetect.com c:\ntdetect.com
Attrib +r c:\ntldr
Attrib +r c:\ntdetect.com

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If you are familiar with DOS, that's the equivalent of :

fdisk /mbr
sys c: