While I don't Have a solution for your backup problems, except buying a 500gb external... I can suggest this:
Download Ubuntu 10.04 LTS desktop from Ubuntu.com. Burn this to a DVD.
Download gParted Live from a Google search. Burn this to a DVD.
Insert gParted Live into your disk drive.
Boot it, and follow the onscreen instructions.
It will open a GUI with a partition manager. You will be able to recognize the portion that is Windows. It will be all one partition, probably /dev/hda1/
If a part under /dev/hda1 says "unallocated space" and is 50gb:
TELL ME HERE.
If it's unallocated under /dev/hda1 (or whatever it is on your laptop, this is probably why it's a "dynamic drive"
If this is true, I will walk you through taking that unallocated space, and moving it out of /dev/hda1.
If it's already out of there, then I honestly don't know why ubuntu won't boot... But try this:
Right click the 50gb unallocated space, and select "delete", then "apply."
Shut down gpartedLive, and insert your Ubuntu 10.04 LTS Desktop DVD.
Try the install with manual partitioning..
Get back to me on this!
Ps: I really hope it doesn't come down to unmounting windows, installing ubuntu, unmounting ubuntu, mounting windows, and mounting ubuntu... But yeah.