I had the same problem befour keep coming every few days boot manager missing.
I Insert windows 7 sp1 CD and repair after few days bootmgr missing again.Then I format the hardrive and install win 7 in few days same problem boot manager missing.
Then replace the hard drive problem is slove.problem was the boot sector on the hard disk had the bad sectors.
I have replaced the hard drive at least once but I still can't boot to the DVD in order to repair or reinstall Windows 7.
I am able to boot into the system now. Bad stick of RAM in slot 2. RAM removed and system booted right up. Put stick back in, system won't boot.