By default Windows sends an administrative alert when the amount of free space remaining on a hard disk drive falls below 10 percent. This percentage can be configured using this setting.

Open your registry and find the key below.

Create a new DWORD value called "DiskSpaceThreshold" and set it to the percentage of free disk space remaining before an alert is sent.

Restart Windows for the change to take effect.

Settings:
System Key: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanmanServer\
Parameters]
Name: DiskSpaceThreshold
Type: REG_DWORD (DWORD Value)
Value: 0 - 99 percent (Default is 10)