BigNick - to answer the question you keep asking - as long as you do things right you won't lose your Win7 data.
1. Resize (shrink) your HD partitions in windows to make space for your BT install. There is a windows utility (I forget the exact name - some sort of "disk manager") that will do this for you.
2. Boot off your BT disk. Type "ubiquity" at the prompt (well, ok, I haven't done this in BT5 yet, so I suppose it might not be the same command - somebody speak up if it has changed).
3. Follow the so-simple-a-windows-user-can-do-it instructions. Just be sure to install BT to the empty space you created in step 1.
4. Reboot. You'll see GRUB loader instead of the Win7 boot process you are familiar with. It'll default to booting into BT (at least mine does) so whenever you want to boot into Win7 you'll have to pick that option during the GRUB loader process.
Voila. You now have a dual boot l33t h4x0r machine. And you still have your Win7 box and data available, just with slightly smaller storage on the disk since part of it is now devoted (and aren't we all?) to BT.