Asus A8N32-SLI Deluxe
AMD 4400
7800GT
And a raptor hard drive...

Nope, I don't have the installation DVD in the drive, can I tell him to boot a selected disk partition somehow?
Or do something from it that would help?
It's not an installation problem : I had a triple boot (10.4,10.5,Vista) working great before that thing happened. As I said, i didn't change anything to the system in the meantime. I know it sounds physical, but I'd stay it is unlikely (just a strong hunch).

I tried diskwarrior from the 10.4 partition (which was still working). Disastrous effects: now I get a chain booting error after selecting Mac os from the vista menu.
Don't know how and if it can be fixed.