Hi Peter,
You have two options in how to install XP on your Medion.
The correct way, I found out after we had done Tonys
Is to press F6 when asked if you need to install extra drivers for SATA or SCSI drives and tell it where you have the SATA drivers. Usually at that point you would have the SATA drivers on a floppy, unfortunately your pc doesnt have a floppy drive so you could use an external floppy or possibly a USB flash drive. I think the drivers should be on the XP CD, probably in system 32 folder but I have never had to do it, possibly they may be available on the medion manufacturers site or even on the Recover partition on your hard drive, look for the Tools folder.
The way we did it on Tonys was to enter the BIOS and alter the AHCI setting to IDE in the part of the bios that I think was for onboard peripherals but may have been a different screen. Maybe Tony can have a look at his bios and tell you which screen it was in.
Once that was changed and Save Changes and Exit was selected it booted off the XP CD and installed as normal.
Someone said that once its been installed that way then you cant use the HDD2Go drive in the recessed bay or anther external SATA drive as its set to run as IDE.
You can however change the setting from IDE back to AHCI once XP has been installed and you should be able to boot from either system as normal and use the bay and external SATA drives.
Normally for dual booting you would install the older OS first then the newer one so it should be XP then Vista, so if you still have problems installing XP after following either of the 2 methods described then you will have to back up your Recover partition to a DVD or external drive then format the drive and install XP and create your partitions when installing it.
Then you can install Vista as the second OS.
Let me know if you need any clarification on anything