If the DDNet service fails then it will put error information into the Event Log (look under Application Log). The Event Log can be viewed in Control Panel -> (Performance & Maintenance) -> Administrative Tools -> Event Viewer).   Look at recent errors given by ddnet and then look up the errors by entering the error in the box above.

A common reason why ddnet service fails is that the DinkeyNetPath is not on the local machine. i.e. the dongle server. This must be the case and it cannot be specified as a UNC pathname or a mapped drive letter. i.e. must be in the form c:\dir.  This is because services do not have the access rights to access network components.