I started using one big partition for everything and separating data out with folders when I got my current computer years ago. I'm preparing to upgrade my system from Windows XP to Windows 7, and I thought I might go back to putting my data on a separate partition.
Most likely I'll just use the default OS install. My current Program Files tree has ~16 GB of stuff.
Thinking ahead though, I've had Windows XP installed for years. Who knows what applications I'm going to install down the line?
This, of course, begs the question: How big do I make my Windows 7 install partition?