Might well be in the wrong place here but here goes..
The problem is my nephew's :-)
He used Partition Magic to partition his HD and installed some flavor of Linux on the new partition (Mandrake I think - unable to check since his system is currently off-line) From what I understand the situation is this:
On attempting to reboot into Winxp, he received the error message - BOOT DISK FAILURE.
Reboot attempts all failed and he had to boot from CDROM using his XP disk.
He seems to think Linux has put stuff in his boot sector and tampered with his boot.ini file.
I'll find out more from him when I am able.
If anybody can shed some light on this it would be very much appreciated.