Try these boot disks and see if you can get into the Recovery Console or see if you can borrow a XP CD and load the Recovery Console normally.
HOW TO: Install and Use the Recovery Console for Windows XP (Q307654)
310994 - Obtaining Windows XP Setup Boot Disks
You will have some access if you can load the XP Recovery Console onto the machine.
If you have to download files from Microsoft to put onto Floppies to achieve this make sure you obtain the right version to match your ServicePack level and version of XP.
310994 - Obtaining Windows XP Setup Boot Disks
Once you have the Recovery Console loaded you may proceed with attempting to fix your error.
Try ChkDsk /r and also have a look at the FixMbr, FixBoot, and BootCfg commands.
An easy to follow recovery console description when unable to start computer due to corrupt registry.
This is a layman's version of Q307545 in simple language.
You might like to try this first just in case it works?
How to Use System Files to Create a Boot Disk to Guard Against Being Unable to Start Windows XP (Q314079)
Q305595 - HOW TO: Create a Boot Disk for an NTFS or FAT Partition in Windows XP
Replacement of CD or Product Keys.
To replace a CD or Product Key, you must contact PSS. To locate the appropriate phone number for your product, visit the following Microsoft Web site:
326246 - How to Replace Lost, Broken, or Missing Microsoft Software or Hardware