The System Event Log will display the following warning when less than 10% disk space is free:

Event ID - 2013

The <disk-letter> disk is at or near capacity. You might need to delete some files.

To configure the percentage of free space the disk should have left before writing an entry to EventLog edit this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \LanmanServer \Parameters]
DiskSpaceThreshold=0 (0-99 percent, Default = 10%)

More Info MS KB112509

Windows 2003 can also write an entry to the Eventlog when the free space on disk is below a certain treshold specified in megabytes:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \LanmanServer \Parameters]
LowDiskSpaceMinimum=0 (Default = 400 MByte)

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