If you've ever run low on disk space on a particular volume, you may have noticed an entry in the Event Viewer, warning you that space is low and that you may need to delete some files. By default, this entry will be written when a volume reaches 90% full. You can change this value by going to the Registry and looking for this string:
In the right pane, right-click on the 'DiskSpaceThreshold' key and enter in a value from 0 to 99, which will set the value at that percentage full. This setting will be in effect for all volumes.