Reboot and select proper boot device

The other day my home-built system's overclocking failed i.e. on restart, it said, 'Overclock Failed!' etc.
So I set it back to default settings, and then it started up, a little slower, but fine. Played Prototype for a while then shut it all down.
Next morning, on startup I get, 'Reboot and select proper boot device' etc. I've reset all the BIOS settings, made sure the HD is the primary boot device, turned off all the other boot devices, and nothing. Even selected the HD as the boot device directly.
I booted from my vista DVD and tried to repair, except that there is no operating system for me to select... its disappeared! So I can't repair, it only wants to do a clean install.
Something which has sprung to mind is that I used to have a partition with linux installed on it, which changed my master boot to use GRUB, I think. Anyway I didn't use it and wanted the space back so I deleted it and reset my master boot record using the vista DVD, typed something like 'FixMbr' into the command prompt, and then everything was fine.
Maybe thats come undone somehow, but I have no way of fixing it again, that I know of.

Hopefully my hard-drive's not completely broken =(

Here are my system specifications;

Intel Core 2 Duo E8400 Socket 775 (3.0GHz) 1333FSB 6MB L2 Cache
ASUS Maximus Formula Republic of Gamers iX38 Socket 775 8 channel audio ATX Motherboard
Hi-Power Black 900W Modular 13.5cm Fan PSU - 4x PCI-E, 20+4pin, 6x SATA
2x Corsair 4GB Kit (2x2GB) DDR2 800MHz/PC2-6400 XMS2 Memory Kit CL5 1.9V
Samsung SpinPoint F1 HD103UJ 1TB Hard Drive SATAII *32MB Cache* - OEM
2x Sapphire HD 4850 512MB GDDR3 Dual DVI TV Out PCI-E Graphics Card

Extra Drives: 2x Samsung SpinPoint F1 HD103UJ 1TB Hard Drive SATAII *32MB Cache* - OEM
2x Western Digital WD2500KS 250GB 7200RPM SATAII/300 16MB Cache Hard Drive - OEM

Used to have the processor overclocked to about 4GHz, definitely stable for months and months, had tested for days with multiple primes etc.

Bought the main HD with the Operating system on back in August of 2008.

Hopefully I've included enough detail, thanks for reading my post and I am very gratefully for any help or suggestions.

Cheers, Charlie.

Did you add/remove any of those (5?) HDDs? Or switch which SATA port they were on? If that was the case, you need to switch everything back to exactly how it was. It's possible you have a bad drive that holds the boot information, but windows is still fine(they could be on seperate disks). Definitely possible that OCing may have corrupted the HDD, but not necessarily the windows drive.

Were all the drives installed when you installed windows?

It may be something as simple as you're booting off the wrong drive. Since you have 5, unplug all but the one that you want to boot off of. Then make sure it's the first boot device. If it's still giving an error at that point, then you would want to do the windows repair.

You may not have been on the correct drive. Make sure you are on the same drive letter that is assigned to your HD.