Well you have me at a disadvantage as I haven't tried this with Mandrake or reiserfs.
One thing to look into tho... The part about using sfdisk and dd to save and restore the partition layout does seem kind of redundant but I read someplace that using the dd tool may be needed in the case of extended partitions.
Everytime I have installed Mandrake, it automatically sets up an extended partition.
With that in mind, maybe you should use dd to restore the mbr...
sfdisk /dev/hda < /mnt/images/backup/winxpm10partfile
dd if=/mnt/images/backup/winxpmd10mbrfile of=/dev/hda bs=512 count=1
EDIT: I just loaded Mandrake 10 on a box and as I expected, it created an extended partition. My partition table looked like this....
/dev/hda5 Linux /
/dev/hda6 Linux Swap
I backed up the partition table using sfdisk and dd tool which is good because restoring the table using sfdisk failed ( because of the Extended partition ) . The partition restore worked using the dd tool so now I will proceed to restore the systems.
I'll let you know if that works ok.