The bios has really nothing to do with Windows 7.
It controls only the motherboard and what options are set there.
Every time you boot up it looks for what hard drive you have installed.
If you go into the Bios during bootup, it will show you all the current drives.
The days of having to tell the bios what drive you have installed is long since past.
If you're replacing an old drive with a new one of the same type, there is NO need to even go into the bios. It will handle the change automatically.
Futzing around in the bios when you don't have to, can get you into all sorts of trouble.
Once Windows starts running, the bios is left behind. From there on all settings like assigning drive letters is done strictly in windows.
Is there a chance that you can reset your bios to the default settings and see if then you might be able to make the changes? I am not familiar since I never had that problem. Just thinking that may work. Also If it is an older computer you might check the battery. I did have a problem at one time and had to replace the battery even though it was different from your problem.