I recently ran into a problem with some code I had written getting a 401 error on a particular server. There were a lot of pieces involved as it was custom .net code calling the excel services web service on a SharePoint 2007 machine.
After playing around with the code on various boxes I simply couldn't find any reason for why the code would not work. I even tried hard coding some credentials into the web service call for testing but still no luck.
I suspected that it wasn't the code, but the trouble was how to confirm that which is when I grabbed Fiddler Web Debugger http://www.fiddler2.com/fiddler2/version.asp .
It's a really great free tool that is very easy to use. I downloaded it to my local machine and tried a bunch of calls to the server. Initially I received a 407 message back.
This told me I was having a problem getting through the proxy, which as the other box was outside the proxy was unlikely to be the cause.
After turning off the proxy settings in my browser I then continued to see 401 messages. This eliminated the problem being code related as a straight http request to the file in question would not work.After the sys admin had a look at the results from fiddler we then looked into the settings in SharePoint and found out that kerberos wasn't working at all.
Problem solved :), I'd definitely using Fiddler if you've got a tricky authentication problem to solve.