Increase the Amount of Space used by System Restore

Open up System Restore:

Start (button in the lower left corner); Right click on Computer, then click Properties.
In the upper left panel select System Protection > Configure*, then adjust your disk space usage here.

*Note Protection Settings must be turned On or you won't see "Configure (in the unlikely chance they are set to "Off", refer to this post to turn them on: http://digital-qa.blogspot.com/2011/07/turn-system-restore-on-or-off.html).