Could it be a possible problem with the MBR?

I was wondering if any one could help me out my ex girlfriends computer isn't diplaying anything on the monitor.

When you go to turn it on it seems to boot up ok, (i.e. no post beeps to warn you of anything) but nothing comes on the monitor, I have tried the monitor on another computer and it displays fine. I also have tried a seperate GPU in the machine still nothing, I took out the HDD to back up the files on my computer, whilst I had it out I tried to boot into it on my machine with no joy it wouldn't even show up in the bios. Though it did show up in windows and thus I was able to back up the files that she had saved.

My question is really this as it wasn't showing up in the bios as a bootable HDD, but was showing up in windows when I booted in to my computer, could this HDD have a corrupted MBR?

Just wanted to double check before I went ahead and recovered it.

Listing as a bootable drive has nothing to do with the MBR. On your system, if it was visible in Windows, it was visible in the BIOS and selectable as the first boot device.

If there is no video on her computer (meaning, you can't see it POST or get into the BIOS), you are going to have to look at the other hardware. I would reset the CMOS first, to make sure there wasn't an odd setting. After that, I'd try using only one stick of RAM and try it in different slots if it doesn't work right away. After that, I'd suspect video card/power supply/motherboard. Lastly, the CPU, because they are incredibly hard to kill, compared to other components.