Hi all,
just wished to report: this issue also happened with me after playing around with CCleaner portable 3.8.0.1475.
Long after being done with CCleaner, I stumbled upon ntuser.dat and ntuser.dat.log (which seem to be truncated files) - but in my case these were located on the root of my E:\ drive, and not on C:\ as others have reported. Both files were promptly deleted without issues.
The folder where CCleaner is running from happens to be located under E:\ drive.
Cheers