System restore will be using nearly 2.5Gb of the "C" drive, reduce this to around 1Gb. Save 1.5Gb
Does your son download video's? If so they are probably in a temp folder. Use treesize for free click here to tell you which folders are holding the data.
Also download and run crap cleaner click here for free. this will clean out all your temp files and more. If you want to save your cookies ( passwords ) un-tick the box for this.