I just installed Win 7 Beta 7000 on a MSI GX630 notebook that already had Windows XP SP3 and UBUNTU. (I abandoned VISTA after discovering there was no simple way to make a Windows image backup without laying out $$$$$$ to upgrade from VISTA HOME OEM to ULTIMATE.)
The Win 7 boot menu displays a list:
EARLIER VERSION OF WINDOWS
WINDOWS 7
UBUNTU
All three OS boot up normally without interaction.
My theory on why this works is because I used the notebook eSATA port to install to an external SATA hard drive. (Actually an old 40 GB Barracuda with a $10 SATA-IDE adapter.) External HDs on a USB port are not supported for installation.
I also use that port to do image backups to another old Barracuda.
I need to do more testing but I think Win 7 does not like to share the first hard drive with another OS.
Now, if I can figure out how to downgrade the USB floppy driver to XP's sfloppy.sys. The Win 7 sfloppy.sys does not work.