Write a batch file to do your building, including unregistering the DLL afterwards, we use:
subst Z: .
vb6 /make %ProjectFile% /out "%LogSuccess%\%PROJECTNAME%_Log.txt" /outdir Z:\bin
IF ERRORLEVEL 1 MOVE "%LogSuccess%\%PROJECTNAME%_Log.txt" "%LogFail%"
regsvr32 /u Z:\bin\Constants.dll
subst Z: /D