Empty space within a partition is not what you need to install Linux. You'll need to free up some space on the hard drive for a Linux and a swap partition. You might be able to do this by moving some files from one partition to another, when the partition is completely empty then you could remove it from the hard drive, making the space available for the Linux partitions.

As to the backingup of the mbr, I suggest a little Google searching. I don't know your level of knowledge, if you even know what the mbr is or not. www.cpqlinux.com/mbr.html+dd+backup+mbr&hl=en&gl=us&ct=clnk&cd=2]Here is a link[/url] that i found with a quick search that might get you started.