To explain your mistake which Felu has corrected:
A backslash (ASCII code 92) in a string tells JScript that the following character is a special character. (like \n for a new line or \t for a tab space) When you want a literally backslash in your string, you have to escape it using a second backslash, like \\. If you don't, you get such errors.
And by the way, if you want it to open in IE and not in your default browser, you can just use:
code:
Opener.Run("iexplore.exe \""+MsgPlus.ScriptFilesPath+"\\Viewer.html\"");
instead of using the full path.
/ 
