goto desktop make new folder, open new folder goto tools folder options, view uncheck hide extensions for known file types,hit apply ok MC.Gadget will turn into MC.Gadget.Rar then right click MC.Gadget.Rar and choose rename then just delete the .Rar.. and it will turn into MC.Gadget
also i suggest (going back to tools folder options, view uncheck hide extensions for known file types,hit apply ok) once its turned into a gadget, go back and change it to hide.. if ya want to hide your extensions.
Hope This Helps.. and like cncb said it works with vista sidebar.