Windows 7: No OS found on disk

Hi,
I've got a P7P55D Pro with an I5 750 and three Sata hard drives. One of the drives is partitioned between Windows 7 Ultimate 32 on C and some general rubbish on the other. I was looking at clocking my memory a little & got to a stage where the system was cycle rebooting so I loaded the bios defaults. The system then went into a startup repair mode where it said 'No OS files found on disk'.
I've unplugged all my other hard drives now except the OS one. Using the recovery disk & DISKPART I could see that neither my C or D partition where showing as active (I assume thats the asterix) so I set C as active as detailed in this help: windows 7 won't boot after changing active partit thread. Now whenever I reboot the Startup Repair starts and seems to make my C partition not active again. I can handle most problems I come across but this has got me totally stumped. Please help.
Thanks,
J

You need a Windows 7 repair disk: System Repair Disc - Create

Boot Repair CD, click through to Recovery tools list to open a Command Line, type

DISKPART
LIST DISK
SELECT DISK # (for Windows 7 HD)
LIST PARTITION
SELECT PARTITION # (for Windows 7 partition, determine by volume size)
ACTIVE
EXIT

Now return to Recovery Tools list to run Startup Repair up to 3 separate times with reboots as it will attempt to repair and finally write the MBR to Windows 7.

If this fails you need to run full Factory Recovery which might allow you to save your files first.

Thanks, thats not dissimilar to what I have tried already but I will try that. I am just running hard drive diagnostics atm.

If you can't access the system, you can download a recovery disc here - Windows 7 tips, tricks, help, and how-to guides

Yeh, I've tried the recovery disk. The startup repair fails. System restore is off, always has been. I would lose a load of files if I recover from a system image (I think). I've done the memory diagnostic which is fine & I've used the command prompt with DISKPART several times. I will try exactly what gregrocker has suggested after this drive diagnostic has finished. 50%through a 1TB sata drive atm but it has picked up 1 error so far. Phew! 2 hours to go. If all else fails I will put the drive in my caddy and transfer the files I need to my laptop & start over. Thanks for your help.

Finally got into windows. The drive diagnostic found one error but didnt fix anything. I ran a chkdsk c: /r /f and it recovered orphaned files then ran through the rest of its job. I then ran startup repair which failed a few times. I then rebooted & it worked. Time for a new hard drive I think.
Thanks a lot.
J