Converting VBScript's MsgBox Function
Definition: Displays a message in a dialog box, waits for the user to click a button, and returns a value indicating which button the user clicked.
Windows PowerShell does not posses built-in methods for displaying message boxes. However, it’s very easy to use the New-Object Cmdlet to create an instance of the Windows Script Host (WSH) Shell object; from there you can use the WSH Popup method to display a message box. The following example first uses New-Object (and the -comobject parameter) to create an instance of the Wscript.Shell object. In the second command, the WSH Popup method is used to display a message box, with the resulting action (that is, the value of the button the user clicked to dismiss the message box) stored in the variable $b:
$a = new-object -comobject wscript.shell $b = $a.popup("This is a test",0,"Test Message Box",1)
For more information on the parameters used when displaying the message box see the Windows Script Host Language Reference.