This is weird, I have two hard drives (160GB and 120GB). I have Windows 7 installed on the 160GB hard drive, but as soon as I remove the 120GB extra, Windows 7 says it can't find the bootmgr (or ntloader, I can't remember). Either way, the computer REFUSES to boot without the second hard drive. There's literally nothing on that other hard drive except games.
I've switched jumpers as well, makes no difference.