Method:  WshNetwork.SetDefaultPrinter

WshNetwork.SetDefaultPrinter strPrinterName

The SetDefaultPrinter method sets the default printer to the printer identified as strPrinterName.

The following VBScript code sets the default printer to \\Server\LaserJet.

Code:
Set WshNetwork = CreateObject("WScript.Network")
WshNetwork.SetDefaultPrinter "\\Server\LaserJet"