I came across this today and wanted to share.  I have looked for this in the past with little success so maybe you will come across this post when you need it.  Sometimes you need the default password that was assigned to IUSR_MachineName and IWAM_MachineName from when you initially installed IIS.  That information is actually quite easy to get.

First make a copy of adsutil.vbs located in c:\inetpub\adminscripts, name it something else, and move it to a folder with locked down permissions:
copy c:\inetpub\adminscripts\adsutil.vbs c:\admin\adsutil_iispw.vbs

While you could certainly put the copy in the same location and delete it when you are done, I prefer to put it in a location that only administrators have access to so I can easily reference it in the future.  It all depends on what your best practice security measures are.

Next edit adsutil_iispw.vbs and go to line 2592.  It currently reads:

IsSecureProperty = True

Edit it so that it reads:

IsSecureProperty = False 

Save adsutil_iispw.vbs and close it.  Remember  that any time you are editing script files, config files, etc., you are best off using Notepad so that extra characters and formatting are not saved to the file which would prevent it from running properly.

Now you can run the following to get the IUSR password:

cscript c:\admin\adsutil_iispw.vbs get w3svc/anonymoususerpass

or the following to get the IWAM password:

cscript c:\admin\adsutil_iispw.vbs get w3svc/wamuserpass

I hope this helps when you need it!