I noticed at work that I started getting prompted for a username/password when accessing web pages running on localhost through Firefox. If I didn't supply my Netware credentials I got a HTTP 401 - Unauthorized response. There were corresponding entries in the event log with Event ID 100 (see below).

The resolution for me was to open secpol.msc (Local Security Policy Editor) and make user IUSR_ had the following rights:
* Access this computer from the network
* Logon as a batch job

I had read somewhere that the account also needed "Logon locally", but I wasn't able to add anything to that item (presumably because of network security policy settings). "Logon locally" contained Administrator and User groups. So, I added IUSR_ to the Users group. I also had to remove it from the Guest group due to another policy that restricted what Guest accounts could do. At that point, I was able to access the pages without having to supply credentials again. I figured all this out from piecing together info from two key webpages, but I don't have the URLs handy at the moment. Maybe I'll post them later in a comment...

This was mainly a problem for me because it caused POSTs that I was doing through VB.NET using the HttpRequest object to fail with a StatusCode of 401.

Event Type: Warning
Event Source: W3SVC
Event Category: None
Event ID: 100
Date: 2008-04-10
Time: 17:08:09
User: N/A
Computer: marvin
Description:
The server was unable to logon the Windows NT account 'IUSR_marvin' due to the following error: Logon failure: the user has not been granted the requested logon type at this computer. The data is the error code.