Easy Geek question

Can anyone recommend a good freeware program for making a constantly-updated
backup of my system? I just got a 200GB external HD, and would like to
install a program that keeps track of any changes I make to my internal HD
and keeps a mirror copy on the external HD, just in case I have another
total system crash like last year. I also want to backup SWMBOs system on
the same external drive, if possible, although hers would be backed up
intermittently (whenever I managed to bring the external HD over to her
office and update).

Thanks, and, uh....TL.

--riverman