"...WinXP clobbers the whole HDD, and never offers to be installed on a partition."

I wonder if that's a "feature" specific to XP Home? I haven't done an install of XP (always Pro so far) where I needed a partitioned drive, but I know at least one vendor (Sony) sets up a default where the OS and user files are on different partitions. So It must be possible SOMEHOW.

If the installer really can't be made to partition the drive or install to a partition on a pre-partitioned one, it might pay to use QTParted to get free space, then "repair" the XP installation before trying to do the Linux install. That should make boot.ini agree with the actual drive geometry--and a disagreement between the description in boot.ini and the actual disk is probaly Windows's problem.