How to create a Windows XP Boot Disk
If there is a faulty boot sequence on the Windows XP machine, then you might need to create a boot disk to access the machine.
Microsoft Knowledge Base Article 305595 provides step-by-step description of how to create a Windows XP boot disk.
See also Restoring Windows Boot Loader Manually.
It may also be a good strategy to think of the preventive measures, so that the next time you face a similar problem it will take you next to nothing to resolve it. See Acronis Backup & Recovery 10 and Acronis True Image Home 2009.