This tip is absolutely and completely useless, and will even make applications that depend on timestamps have issues. It's mostly just to be used as a cool trick you can show off to your friends, so we're going to show you how it works.
The way this trick works is by modifying the regional settings for the time format. Instead of using just AM or PM, you can put up to 12 characters of text in that field.
Start by opening Regional and Language Options from the start menu search box or the control panel.
On the Formats tab you'll see the "Customize this format" button, which you'll need to choose.
Select the Time tab, and then here's where we can make whatever settings we want.
I chose to set mine to "AM - Geek" so I could still see AM or PM, but you can choose any 12 characters for either field. Keep in mind that the PM symbol is only going to display during the "PM" hours.
This does change more than just the displayed clock, so I'd advise not using this on a computer you do actual work on. Using the F5 key in notepad shows the "Geek" text as well:
It's one of those things that's interesting to know… just not very useful. It does work in XP as well.