You could use Partition Magic to create a partition containing your existing data and leave the rest of the drive as "free space". Then when you start the Red Hat installer it should offer you various options for partition setup you will need the one which says "keep all existing partitions and use free space". I would suggest that you backup your backup first though just in case. Space required depends on how much of the Linux install you intend to use. A full graphical interface will require far more space than a text only setup. Red hat can require about 2 to 3 GB for a full setup but allow far a minimum of 4GB if you want every included package.