"Is there something in WinXP which won't allow two copies to live on the same machine, even if only one is on the boot drive?"
No
I've used Acronis Driveimage, HDClone, EASEUS Disk Copy and DriveImage XML all successfully.
Usually a bit of faffing around, but it beats a full reinstall.
Good overview here:
Free Clone Hard Disk Software to clone your Hard Disk data
Mac