I think I'm going to try an SD card. It's what I've already got, and the CDs
aren't big enough. But even the SD card is only 16gb, and my home partition is
25gb. Will I be able to back them up individually? That way I can save the
important ones. Or is it saved in one block?

"Backup important files in your Wubi /home first. They SHOULD
be safe after your partition and install, but there is always the
possibility that something can go wrong. If you have nothing there
you don't mind loosing, go ahead with the install. Otherwise back up
first. The process to mount and back up is the same from a live CD as
it will be from a new install."