Interesting post going over the process to use Log Parser to find out on the Exchange logs how many clients are still using Outlook 2003.
I have had many customers work their way out in pulling the information of what client is being used by various users in their Exchange 2010 environment. Especially Outlook 2003 clients. As if you have Outlook 2003 in your environment, you will not be able to go with Exchange upgrade to a new version.
Outlook 2003 is not supported with Exchange 2013 or even Office365.
One of the most easiest and fastest way of getting that information is by parsing the RPC Client Access logs using Log Parser Studio (LPS) tool written by Kary Wall. You can know more about what are the updates to this by adding yourself into RSS feeds on this http://blogs.technet.com/b/karywa/archive/tags/log+parser+studio/
Read more at source: http://blogs.technet.com/b/messagingninjas/archive/2015/05/12/pulling-out-users-who-are-connecting-using-outlook-2003-in-your-exchange-2010-environment.aspx