1. Open an elevated command prompt, as follows: Go into Start -> All Programs -> Accessories, right-click on “Command Prompt”, click on “Run as administrator” and click ‘Yes’.
2. Type in the following and press enter: powercfg -h off
3. Close the command prompt.
Disable system restore:
1. Go into start, right-click on “Computer” and click “Properties”
2. Go into “System protection”
3. Click on the OS drive (usually ‘C:’) and click “Configure…”
4. Tick “Turn off system protection”, click ‘OK’ and click ‘Yes’.
Run a full defrafmentation with Defraggler (link) or any defragger tool that consolidates the space. Defraggler is a freeware tool which consolidates the data during its defragmentation process, leaving the free space at the end of the drive. This is necessary to be able to be able to shrink the partition further. The Windows own defragger does not usually consolidate all the data.