Believe it or not - you may have too much free disk space!
I've just had a look through Google on this one, and it would appear that many people (including officials from Microsoft) are suggesting that to install it, you need to start filling your hard drive up to brin it below 4Gb free:-
From the look of it, I would just suggest copying a few folders into a temporary holding area to bring the free space down, then once you have outlook on delete the copies..
Remember in the computing world, nothing is ever unique - especially when it comes to Micro$oft!!! :-)