What's the best open source Windows back-up app?

I have a 250GB external hard drive which I use to back-up my Windows XP machine. Sorry, Linux fans, but there are apps I need to run that are Windows only, otherwise I'd switch.

Point is, I'm fed up of copying files manually. So, what do you guys recommend as a solid, open source back-up utility that will copy my Documents folder and then, on subsequent runs, only copy over updated files.

Ideally, something like Mac OS X's Time Machine that'll allow me to restore files based on date. But at the very least something that'll automate the work.