I was attempting to add a post to a thread here about changing the boot order. Since I was on my MacBook at the moment, I turned on my Acer to put it into the CMOS menus. I then paged over to "Boot" and left it there while I was writing my post on my MacBook. Suddenly, the Acer screen went black. I then tried to turn the computer back on and it came up with a menu option to... either "repair" the computer or start Windows normally. I selected the latter and the start aborted to a black screen again. Up to this moment, I was on the battery. I then plugged it into the wall. The next time up, I selected "repair", thinking all kinds of bad thoughts about maybe having to reinstall Windows, etc. The computer "ginned" along for several minutes before a screen popped saying the computer could not be repaired and would I like to "restore" the computer to an earlier time when it was working. I selected that and after several more moments, it popped up back in a normal Windows 7 operation. This was the first time I could see the battery status which at that moment reads 9%. So, I assume maybe the battery got down to 5% when I was lingering in CMOS and then did a shutdown which screwed up something in the ability to boot into Windows. From this, my advice would be to ALWAYS BE POWERED from external power when messing around in CMOS unless you know that you have several minutes left with the battery.
