The main reason for not being able to shrink the disk are that there are unmovable files on the disk at the time of trying to shrink the volume (as your screenshot says).
The most common "unmoveable" files are files which are locked during normal computer operation such as virtual memory/pagefile/system restore files as well as a few other files which may be open, but not running "in memory"
Having come across this myself previously on both server and desktop operating systems - I can say the most likely culprit is the pagefile.
To fix this:
- Right-click Computer
- Select Properties
- Select Advanced system settings
- Select the Advanced tab and then the Performance radio button
- Select the Change box under Virtual memory
- Un-check Automatically manage paging file size for all drives
- Select No paging file, and click the Set button
- Select OK to allow and restart.
Once your machine has rebooted and you know you have no page file (check at the root of C: with hidden and system files showing) - try a defrag and then try shrinking the volume again.
Don't forget to reset your pagefile back to its original size afterwards! Failure to do so will potentially cause significant performance issues with any machine.
Hope this helps.
EDIT: I have just had to do this on another system which had other unmovable files. These turned out to be shadow copies of drives on a 2008R2 server. To remove these, use the following command in an elevated command prompt:
Vssadmin delete shadows /For=:<driveletter> /all