However, as mentioned elsewhere in the forums, while free space contains information, there are other parts of the system which also contains copies of files which users may not be aware of. While CCleaner helps to alleviate this to a certain extent, nothing beats vigilance and caution... And because of how Eraser works (in the previous post) the portion which was erased changes at every run, so running till 50%, cancelling and running another 50% will not sanitise the complete free space area.
The best way to check any erasure is to use a file recovery program. I use and recommend Recuva, but there are many others. In your particular case, what really matters is not necessarily the amount of space that was erased, but whether recoverable sensitive information remains. If it does, the problem may well be more the result of the issues mentioned by Joel than failure in the erasing as such.