|I haven't used disk imaging software, but I hear Acronis is awesome for this task.|
When I want to backup, I plug in my external drive and copy over the My Documents folder, which is where I save pretty much every file I create. I then look around other directories for files/folders I might want to save. Keep in mind, if you're not using disk imaging, you'll need to reinstall your other programs too, so the Program Files folder in it's entirety isn't something you'll probably need to backup. Some programs save data or game files there, so it's a good place to look.
Also remember that backup drives can fail too, so make sure you backup really important data on multiple mediums like DVDs.
I'm curious, why are you looking to reformat your hard drive? Maybe it's a problem we can remedy and save you the time of reinstalling your OS and all your applications.
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.