This can be done using Javascript code. Hence if the viewer has disabled Javascript on his blog this may not work. Also many viewers have pop-up blockers in place which may also prevent the display of this mesage.

To add it in your blog follow these steps :

1. Login at Blogger.com.

2. On the Blogger Dashboard which opens for a successful login click the 'Layout' link.

3. The Page Elements subtab of Template tab will open.

4. Click the Edit Html subtab of Template tab.

5. On the Edit Html subtab scroll down in the Template Code box.

6. Copy the code below :

<script language='JavaScript'>
//
alert ("This is a Demo Pop Up Window where you can put any Messages to viewers!")
</script>

7. Paste the code above just before </head> and in your template. Replace "This is a Demo Pop Up Window where you can put any Messages to viewers!" with your own message.

8. Save the template.

9. Clear Cache and View Blog.