That's why you get them to sign a waiver. Why would you want to guarantee somebodys data? That's asking for trouble. An installfest should be all care and no responsibility. Check out this waiver:
Legal responsibility is all well and good, but it doesn't alleviate the problem of 'I tried Linux once, and it trashed all my files'; this is NOT a view of Linux we want to promote. It might be an idea to dump the any essential user's files (Memoirs, Half-finished novel, last 7 years tax reports, etc.. not their 17G pr0n collection) onto a samba share before trying a repartition. In the quite unlikely event that disaster strikes, I would be prepared to offer a free reinstall and recovery of Windows after the LUG if anything went wrong. Presumably they still have the CD's that the software came on, and if not I'd be suggesting Free alternatives :) BTW; for backing up Windows files I personally boot KNOPPIX, mount the filesystem(s) read-only and cp -a or tar onto a network share. I have no idea how well (badly) this preserves Windows concept of file permissions. I don't normally aim for a 'bootable' image because I'm usually backing up for a reinstall anyhow.