How To Use the Recovery Console on a Windows Server 2003-Based Computer That Does Not Start
Are you an Administrator User in 2003, if so, try that systems Recovery Console?
A Discussion About the Bootcfg Command and Its Uses
You may need this article to get into the XP drive from 2003.
How to add more power to Recovery Console by using Group Policy in Windows XP Professional
When you are eventually back in XP go into Group Policy there and Enable the following policy via Local Computer Policy/ Windows Settings/ Local Policies/ Security Options
Recovery console: Allow automatic administrative logon
This security setting determines if the password for the Administrator account must be given before access to the system is granted. If this option is enabled, the Recovery Console does not require you to provide a password, and it automatically logs on to the system.
You could also (when back in XP) Reset the Built-in Administrators password (don't if he has used File Encryption) by logging into XP as an Admin and typing "Control userpasswords2" in the Start Run box and look for the Reset Admin option (bottom of resulting page)
Other options beside a repair install, these will tell you what files to copy across to the System Partition if you want to make a permanent booting repair.
HOW TO: Edit the Boot.ini File in Windows XP (Q289022)
HOW TO: Create a Boot Disk for an NTFS or FAT Partition in Windows XP(Q305595)
How to Use System Files to Create a Boot Disk to Guard Against Being Unable to Start Windows XP (Q314079)
If they don't work you could try repairing windows by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)