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There are several auto-login hacks out there which makes it easier to login. However, it's a security issue (and it's much harder to stay current on upgrades to XOOPS, especially those due to security issues). Cookies are passed from computer to server in clear text (as are logins), so that info is vulnerable.
Remember, too, there is however the standard Remember Me function in XOOPS using custom PHP sessions (configurable in XOOPS, but requires PHP 4.2.x+). In SysAdmin | Prefs | General, turn on custom sessions and set a long time (10080 is one week, 43200 is 30 days, etc.). Then it's up to the user if they want to use the Remember Me function (make sure it's not commented out in the login block and/or the user.php page).