I use Dantz Retrospect professional.
I have the kids PC with a cheap 120GB USB disk on it which I use as a server, and my laptop has a 'client' agent, 2 of which come with the software.
It automatically backs up the server and the clients with weekly and incremental backups, recycles to clear out files etc.
All for about £80. Needs the machines to be networked, if you have more than one.
BW