If both machines are on line then one scheme would be to use a media sharing site like mediafire, rapidshare, etc. Upload the stuff on one machine and download from the other. You can then delete the stuff on the media sharing site if you want. Even using those sites for free, 100GB won't take that long (although I do suggest mediafire since it's a bit faster than some of the others and less obnoxious).
To use an External HD or Direct connection, click on [ ▶ Manually migrating ] at the bottom of > Switch 101: Migrate your Windows files or system to your Mac