The error occurs due to improper boot order. When pxe is located after the hard disk in the boot order all is well until just after the partitioning reboot. When it starts up again, it tries to boot off the hard drive now that there is a good partition and fails generating the 55AA code. Change your boot order so pxe is first (or floppy if you are using the disk based install) and restart the machine from the formatting phase. It should pick right up and roll.
!55AA is the memory address on the disk where the mbr should be found by the way.