Windows can't be tweaked to show seconds. 3rd party software is must
Use
*www.rcis.co.za/dale/tclockex/index.htm - Works under XP, but may not work well with Windows XP themes as it cannot display color schemes of XP.
Second is *www.drive-software.com/windowsclock.html - With almost 100 themes for displaying Clock but can't use Windows style.