The easiest way is with the Microsoft migration kit. If you're buying a clone or making a BEVS 120GB, then you won't be getting one. If you know anyone with an in-warranty elite (even if they opened it, as M$ won't know), you could ask them nicely to call up Microsoft and get a free kit.
The way to do it without the kit is to hook the drive up to your PC via the sata interface directly on the hard drive (opening up the shell), or with an external connection tool (migration kit cable, Datel XSATA, etc.) From there, you can try copying over the files from the old partitions, and copying them over to the new ones. The programs that you do it with (xplorer360, XPort 360) have been known to hang on certain files though, so you'll have to be actively watching the progress, and dragging over a few folders at a time while identifying problematic files.