You can put customized text in the Windows taskbar to the right of the time. To do this, open the Control Panel and select "Regional and Language Options". When this opens, click on the "Customize..." button and then select the "Time" tab in the new window. You should see fields called "AM Symbol:" and "PM Symbol:". They probably have the text "AM" and "PM" in them. But you can change this to anything you like and this text will then show up after the time in the taskbar.

Once you make a change hit the "Apply" button, then "OK", then "Apply" and "OK" again to save your changes and have them show up. This works in both Windows Vista and XP.