I have been having the same exact problem on a Dell C521. Ever since I installed Vista, it wouldn't boot once a week or so saying "missing operating system." I would boot with the Vista DVD and do a "Startup Repair" and it would fix the boot sector and away we'd go for another week or so.
Finally after 2 months the "Startup Repair" wouldn't work saying that the MBR (Master Boot Record) was missing. Foolishly, I reinstalled Vista, which wiped my drive WITHOUT WARNING. XP never did that.
Anyway, the Dell diagnostics show no problems, BIOS, hard drive, motherboard all come up clean, so Dell won't have anything to do with it since I didn't buy Vista from them (its Vista Business not retail if that matters).
I reinstalled 5 days ago and haven't seen the problem again, but I will thoroughly test before I put my family back on, along with a very comprehensive backup system. Will also check for BIOS updates. Have you or anyone gotten any resolution here? Two C521 systems from Dell having the exact same problem does not seem like coincidence.
Best of luck