A Notepad popup is a text window which is displayed by a HTML email message or Web page using the Windows Notepad utility. Click here for a sample popup. (If you are using Internet Explorer for your browser, the demo should have already popped up.)
Would you like to know more? Thanks PivX. There's a follow-up post to this issue over at BUGTRAQ, which states that this is not limited to just Notepad executing, for now the only fix is to change to another browser.