From the Dept of FWIW

RE: Dual boot Windows-Ubuntu OS.

In past, had a problem when ubuntu update corrupted GRUB resulting in a
'null boot' system. Was a bit of hassle and long delay to fix. Decided
best to keep Widows OS independent of Linux.

Machine BIOS allows a USB drive to be part of boot sequence.

* Loaded Linux (Ubuntu 8.04.1) on an old IDE HDD via a IDE-USB interface
with power switch.
* Changed boot sequence to check for USB HDD before SATA drive.

If USB HDD power ON, boots to Linux. If OFF, boots XP.