Sorry but it's going to cost you another blank CD.
NO laughing from the peanut gallery here because the easiest way to write the correct XP MBR is with a Windows 98 boot disk.
I'm dead serious, I know the user is running XP and I know that DOS can't access a NTFS partition but like I said, I have done this many many times.
I even have a boot disk ISO already uploaded from the last person who needed to do this.
This is one of my SkyDrive accounts, download "W98Boot.iso" Right click and download.
https://skydrive.live.com/redir?resid=7245CB5F0CB0F60E%21115
Burn it to a CD like you did with the other ISO. Don't be concerned that you don't see anything on the disk, the floppy image is in the boot sector of the CD where you can't see it.
Boot the system with it, on the first menu arrow down and select option #2 "Boot From CD-Rom"
Next screen arrow down and select option #3 "Start Computer Without CD-ROM drivers"
At the A> prompt, type: fdisk /mbr
You susually will not see anything happen unless you hit an error.
When it returns to the A> prompt wait a couple seconds for good measure, remove the CD and restart the system by holding down the CTRL and ALT buttons while pressing Delete
Did Windows boot?
Dave