Intermittent RC1 boot failure & missing fixed drive- any suggesti

I have a system with XP (D on a 160 MB disk and Vista (C on a 250 MB
disk. When the machine boots, 90% of the time, Vista cannot be found. When
this happens, I can re-start and boot "into previous Windows OS". When I
cannot boot into Vista RC1, the C: drive is completely missing - it's not
present in the device manager but is present in the the BIOS. All drives are
SATA.

The boot sector *is* on the C: (Vista RC1) drive, which makes this
confusing. I have reformatted the C: drive and re-installed Vista RC1, but
problem continues. I've also

If I reformat C:, copy D: to C:, and install Vista RC1 on D:, D: disappears.
Both drives work (i.e. don't vanish) in my other machine.

If a drive disappeared from the BIOS, I'd understand what's going on. Any
suggestions?

Thanks all!