Here is a link to the useful, and free, SharePoint Portal Server (SPS2003) utility, SPUserUtil.
Keith Richie originally developed this suite of utilities a few years ago whilst working for Microsoft. Though it is no longer supported by Keith or, as it never was by Microsoft it does contain very comprehensive documentation.
Unfortunately this suite is very difficult to come by these days so below is link to a copy. As always, use at your own risk.
Here are some relevant links to Keith’s old blog which contain some additional information.
Also, here is a good blog article on using SPUserUtil by Marcus Hass.