Dual Boot - OS missing when Vista DVD removed

> Hi there, hope someone can help with this:
> I've installed Vista Beta 2 64-bit edition on a second hard drive. When I
> boot with the Vista DVD in the drive (not pressing any key to boot from the
> DVD) I get the Vista boot loader and can boot from my main hard drive into
> XP. However, if the Vista DVD isn't in the drive, I get an 'OS missing' error
> and can't proceed. Will repairing the boot sector with FIXBOOT work? Please
> help!
>
> Thanks in advance,
> Andy
> UK