You only have one drive. It is partitioned into two logical drives, C and D. D is probably just the recovery partition and when you wipe the drive to install 7, you can delete both partitions and freshly install the OS using the full 320GB (or really, it's only 298GB, 1000 vs. 1024 )

As for moving everything between computers, you have a few options. I would suggest going onto Newegg or Tigerdirect and finding a decent external enclosure, taking the hard drive out, and putting it in the external case. That way you have an external drive you can safely take back home, copy all of your music and documents and all to your other laptop, and be on your way to installing 7 when you get back. Just make sure you get a 2.5" enclosure and not 3.5", as those are for desktop drives.

One other way would be to use flash drives or DVDs, pending how much you need to back up, and if you have a burner on the laptop.