If you receive insufficient memory errors on Windows the first thing to check is where you installed ZBrush.
Please ensure that you have not installed the software to the ProgramFiles(x86) directory.
That directory is for 32-bit applications and any program installed there will not be able to properly function as a 64-bit application. This means that it will not be able to make full use of your system RAM.
Since ZBrush and ZBrushCore are only available as a 64-bit application it should never be installed to ProgramFiles(x86). It can be installed anywhere else on your computer, although we recommend the default Program Files directory on your C drive.