How do I free ghosted drive letters?

Can someone help me by pointing me to the proper way to free “ghosted drives”
under Windows Service mode?

My server has drives G: thru Z: free under console windows. When I run a
Windows Service program using the -debug switch under cmd console window, it
can map a remote UNC resource to a local drive letter. However, when run as
service mode, it reported no free drive letters available. It seems that
drive G: thru Z: are “connected” yet not available and I was unable to free
these “unable drives” as they are still mapped. Rebooting the server does
not help!

Any info will be deeply appreciated.

Jeff