It show the following message
http://i.imgur.com/hpTtv.jpg
but look at this
http://i.imgur.com/Ipysf.png
A quick google search tell's me the 4.1 update is around 350~ mb
Another google search solves the problem. Here's a blue quote
"The 4.1 patch will require about 9 GB of free space in order to be installed. Even though it's only about a 320 MB download, the space is required for the patch to unpack itself and the installed game files. Then it applies itself and finally packs everything back up"
Yeah...that's convienient
EDIT: A 3rd google search solves the above problem. Delete cache, update wow, ???, Profit