After one week with some late nights I finally managed to boot my new dual boot Ubunto-WinXP on my SATA RAID-0 machine (silicon image 3112 raid controller). I am using the Windows bootloader.
I started off using Partition Magic under windows to prepare a partition for root and boot and one for swap. I then booted the ubunto cd (well actually the Linux Format provided DVD in my case).
After much booting and rebooting and reading what seemed like hundreds of web pages (and accidentally reformatting the wrong partition and loosing my windows programs I had on a seperate partition), I enventually got started on the FakeRaidHowto.
What confused me to start with was that I had to use the mapper device, i.e. /dev/mapper/sil_afafejbgajdg in my case. There is no conventional device to use. The /dev/sda and /dev/sdb points to the individual striped harddrives and should NOT be used anywhere.
The FakRaidHowto on https://help.ubuntu.com/community/FakeRaidHowto
Mentions the warnings you get about locale. You can generate the missing locale with the locale-gen command if you like, but it's not critical.
The other thing I had to do was to install the checkpolicy package to be able to successfully install dmraid.
The last problem was the grub installation. In order to be able to create the .bin file for the windows bootloader:
dd if=/dev/mapper/sil_afafejbgajdg of=/mnt/floppy/ubuntu/bin bs=512 count=1
I had to install grub to the MBR first. I then created the .bin file as above. I forgot at first that you have to use the main harddisk device and not the partition you are booting from (I have my ubuntu boot on /dev/mapper/sil_afafejbgajdg2
Ones the file is on the floppy I unmounted and rebooted into the WinXP install DVD and run the FIXMBR command from the repair console. Then booted into Windows and set up the boot.ini as other people have described.
So after that it booted, although with the little error 15 message. I guess it's just for me to remove "savedefault" from menu.lst. Then I presume I will have to install nvidia graphics drivers before I get my nice gui up and running....
And maybe I'll try to undo that accidental format of my windows software too.