Macbook says "Startup Disk is full" but it isn't full

For the past couple of days, I've been getting the message that I'm unable to save/download something because the startup disk is full. It says I have about 81 MB left (I believe that's the number, last time I checked) but the bar is completely filled with yellow. Which is strange because I used to have an awful lot of photos/videos up until I deleted them earlier because of this issue... the bar also says I have 0kb for every other category.

After, I downloaded a virus scanner off of the app store to check if it's caused by a virus, but it hasn't had any results. Am I doing something wrong, or is my computer really infected?

I'm pretty much desperate for help at this point, so any links/tips would be greatly appreciated.

I'm not the best with computers either (I'm fourteen) so please break it down for me [I do have prior knowledge of computers, I'm slowly learning more about the Macbook, as I've had it since September 2011]

Here's information about my Mac:

Mac OS X

Version 10.7.4 (It won't let me update due to the full disk)

Memory 4 GB

24.5 MB free out of 319.21 GB

http://tinypic.com/r/116o178/6 <--- View that for a better look on my storage