Question about booting linux from cd or flash drive.?

If I did this would the current os be overwritten and can I use space on the hard drive without making a new partition for it? I'm asking this because I'm dual booting W7 and Ubuntu right now, and I want to try out Backtrack but don't want to mess up my current installation. And I want to know if I can use the hard drive space on the computer because I need to decide if I should boot from an external hard drive or if I have to partition my drive. Thanks to anyone who actually helps me.