That error seems to be coming from the gm module, at
https://github.com/NodeBB/NodeBB/blob/master/src/image.js#L18 and
https://github.com/NodeBB/NodeBB/blob/master/src/image.js#L23, not sure how compatible they are on windows, you can try commenting out those resize lines and see if uploads work.
I commented out those lines and now I get this:
Have you tried running node from a command prompt in Administrator mode ('Run as Administrator')?
I RDP in as Administrator (not very secure, I know, we're going to disable it before going live), so the command prompt runs as admin every time. I tried changing the permissions of the folder it says in the error, but that didn't change anything. I even ran a fresh install of NodeBB, but that had no effect either. At this point what we're gonna do is just run it on a small Linux box.
I can't blame you; Windows can be pretty finicky sometimes. If you have UAC enabled, I think you might still have to specify that you want to run your command prompt in Administrator mode. I'd double check that just to make sure.
I found someone who was having somewhat similar problems with node's GraphicsMagick (gm) plugin on StackOverflow and it looks like their solution involved changing require('gm').subClass({imageMagick:true}) to require('gm'), as well as running node from Administrator mode.
Well now all I get is a "spawn ENOENT" error, which I fixed on the Linux machine by installing imagemagick through apt. Unfortunately, installing it on Windows didn't work.
Looks like a permissions error, but their solution is to use require('gm').subClass({ imageMagick : true}).
Maybe Linux is the answer for you after all! 
Yea, I'm just gonna stick with hosting it on the Linux server. Thanks anyway!
I'll try to reproduce the error and see if I can't fix it.
I've managed to reproduce the issue on Win2k8R2. It's coming from ImageMagick not being installed/configured correctly.
These are the steps I took to fix the issue:
Go out and install the latest version of ImageMagick
After running the installer make sure that you close the command prompt from which you are running NodeBB.
Open a new command prompt and rerun node app.js
The problem should be fixed! 
