What Stilleto said. You're basically cleaning out the garage. You've got boxes you want to stack nicely, a bunch to throw out, and a bratty neighbor kid (AOL) who likes to ride his skateboard in and out as long as the garage door's open. It doesn't make a lot of sense to start out sweeping around the boxes you're throwing out, does it?
Same thing. First you throw shiat out, then you neatly stack (defrag) only what you're keeping... and tell the neighbor kid "Hey, I'll give you five bucks to help me tote this stuff to the curb" which should make him go away and let you work in peace.
Computers do make sense, sometimes.