This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
I haven't done this using JS alerts, but make sure that the character encoding of the web page is the same one that you use for your accented characters. Your headers should include something like <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />, or maybe UTF-8, depending on your circumstances.