If you are using a back up program as well as system restore...the back up files do not show the disc size that are being used... The system just disappears this information....

do a sytem clean-up and remove all but the last restore point...Then turn off your back -up program and clear the cahe to remove the stored files...then reset the back ups and you should see an increas in disc space...

Some anti-virus suite programs such as McAffee also create large hidden back up files that will delete available disc space.

You might also fo into folder options and uncheck the hide files and folders option ...just be careful about what files show up and which ones you are deleting!