Obtain the IUSR account password:  cscript.exe adsutil.vbs get w3svc/anonymoususerpass
Obtain the IWAM account password:  cscript.exe adsutil.vbs get w3svc/wamuserpass

Set the IUSR account password:  cscript.exe adsutil.vbs set w3svc/anonymoususerpass "password"
Set the IWAM account password:  cscript.exe adsutil.vbs set w3svc/wamuserpass "password"

If the password appears as asterisks, edit the adsutil.vbs file and

change

IsSecureProperty = True

to

IsSecureProperty = False

Reference:

See PRB: Configured Identity Is Incorrect for IWAM Account for details.

Example:

C:\Inetpub\AdminScripts>cscript adsutil.vbs get W3SVC/anonymoususerpass

Microsoft (R) Windows Script Host Version 5.6

Copyright (C) Microsoft Corporation 1996-2001. All rights reserved.

anonymoususerpass               : (STRING) "mypassword"