By default, your login.xml file is located at
<drive>\Documents and Settings\<your Windows username>\Application Data\ESRI\ArcWeb
If your Application Data directory is not visible using Windows Explorer, open Windows Explorer and browse to the \Documents and Settings\<your Windows username> directory. Under the Tools menu, choose Folder Options to open the Folder Options dialog. On the View tab, select Show hidden files and folders and click OK. You should now be able to view the login.xml file in the Application Data directory.