New challenge!

So.. i'm faced with a new challenge and that is to get from every single wss-site and portal all the users that belong to a certain domain and put them all in an excelsheet (or more desirable is to check if a new account exists and add this to the site/portal). Sounds cool huh? ;) Update : the requirements are to create a list of all the accounts from the 'old' domain for auditing reasons. And furthermore, every day a batch of 50 users are migrated from the old domain to the new domain. And I've got to check whether one of this 50 users exists in one of the Sharepoint sites and if so, add the new account. After the moment that all the users are migrated I've got to make sure that all of the old accounts are removed from Sharepoint. I'm already playing with it and i'm just a bit confused with function to use from the Usergroup webservice. Should I use :

• usgrp.GetUserCollectionFromSite
• usgrp.GetUserCollectionFromWeb
• usgrp.GetAllUserCollectionFromWeb

I reckon that the last function will gather the same information as the following page "/_layouts/1033/siteusrs.aspx", basically giving me a list of users who ever accessed the site. And the other will provide me with the usernames who are registered as users on that site.