Can't install Snow Leopard - Macintosh HD cannot be used to start up your computer

Hi,

I'm trying to install Snow Leopard, but when I put in the installation disk and try to install, I'm quickly stopped from installing when I get to the screen that says "Select the disk where you want to install Mac OS X." The only option is "Macintosh HD," but there is a yellow triangle on it and when I click on it, it says "Mac OS X cannot be installed on 'Macintosh HD,' because this disk cannot be used to start up your computer."

I do also have Ubuntu running on my computer. I'm attaching a screenshot of Disk Utility so you can see how my hard drive is partitioned.

I don't really understand why it can't be installed there -- Mac OS X is the disk that starts up when my computer starts. I was researching a bit and found it might be a problem with the partition scheme -- but if it is I don't know how to rectify that or know where to go from there.

Ideally I do not want to have to reformat my hard drive or install Mac OS X from scratch, as I will then have to re-install Ubuntu as well.

Can anyone help me out? Thanks!

 

It's GUID, which I believe is what it's supposed to be, right?

 

According to Apple you only need 5GB. And, as you said, I'm sure it would tell me if there weren't enough space.