It's a syntax error caused by this code :
onClick='MyWindow=Window.open('advent.html', '', 'width=300, height=300')'>
You should be writing this:
onClick="MyWindow=window.open('advent.html', '', 'width=300, height=300')">
Have a look at this link for more help with javascript
http://javascriptkit.com/javatutors/window1.shtml