Maybe my original question wasnt clear.
The disk in question was originally an XP system disk, and I was booting from the other disk with Win 7 installed.
I did physically replace the drive (old XP), but because it had the "boot" folder etc on it, the PC would not boot into Windows.
Issue resolved by booting from CD and installing Win 7 on new disk.
Oh boy wish it was as easy as it used to be in VMS
But thanks for the replies.