things in /tmp are up for deletion usually. normally best to drop down to a console (i.e. no X) and stop any available services just incase.
as for your home question... that's why you should have a seperate /home partition. that way you can format your root partition totally, but the old /home data won't be touched at all. if you do not have a seperate home, then i don't believe there's any nice way to help. just take a backup to CD or something beforehand. Most "upgrade" or "repair" instalations won't actually format anything, so you could well get a damaged system back up without a full reinstall.