Currently Being ModeratedSep 28, 2011 4:43 AM (in response to Mo-ree)
Memory is RAM. I think what you are referring to is disk space.
Once you have backed up all your own files to the external hard drive you can delete them from your iMac (don't try to do them all at once!).
That will free up all the space for your daughter's use.
Currently Being ModeratedSep 28, 2011 4:57 AM (in response to Mo-ree)
I would only do a clean install if needed as it usually takes quite a time and does not provide any advantages.
After being done with your backups, you could get along with creating a new (admin) user for your daughter, then log in with her user and just delete your old user.
This will make sure that any of your data that might be stored in your user account will not be exposed to anybody else. I do not mean not to trust your daughter but why go for a risk here?
Currently Being ModeratedSep 28, 2011 7:24 PM (in response to probe_droid)
Be sure to empty the trash.
For more extreme privacy,
Macintosh-HD -> Applications -> Utilities -> disk utiltiy
Use disk utility to zero out unused disk space. one pass should be enouth.