Are you including the Vista operating system as part of this 20 GB? You don't remember how much free space remained after you had created the additional account for your son?
You've tried windows own Disk Clean, I don't suppose you will have been able to defrag, not enough free space for defrag to run.
Could suggest a few utilities to account for and perhaps make sense of what's on your hard drive, but they'd fill it up even more at the minute, things like WinDirStat or Jam Software Tree Size Free.
Have you tried running CCleaner in its default mode?
To free up some space quickly if you have system restore running, you can reduce the windows default size allocated for SR, which I think is 12% of free drive space up to 150GB, which in your case may be about 8GB reserved for SR, which could be reduced to about 2GB or less.
Also try reducing the default size allocated for the Recycle Bin