Код:
  <head>
<SCRIPT LANGUAGE="JavaScript">
<!-- hide from old browsers
function PopIt(label, msg){ 
// Set up Page Colors & Table 
var s1 = "<TITLE>Информационное окно!</TITLE>" + 
"<BODY onBlur='self.focus()' BGCOLOR='ffffff'><TABLE BORDER=0><TR>" + 
"<TD WIDTH=90% HEIGHT=90 VALIGN=TOP ALIGN=LEFT>"+ 
"<FONT SIZE=4>" 
var s2 = "<FONT COLOR='FF0000'><B>"+label+"</B> </FONT><P>" 
var s3 = "</TD><TD WIDTH=10%> </TD></TR><TR><TD> </TD>"+ 
"<TD VALIGN=TOP ALIGN=RIGHT>"+ 
"<FORM><INPUT TYPE='BUTTON' VALUE='Закрыть'" + 
"onClick='self.close()'>" + 
"</FORM></TD></TR></TABLE></BODY>" 
popup = window.open("","popDialog", "height=160,width=300,scrollbars=no") 
popup.document.write(s1+s2+msg+s3) 
popup.document.close()
}

function popHelp(){ 
PopIt('ВСЕГДА В ФОКУСЕ!', 'Кликните где-нибудь вне ' + 
'popup окна.')
}
<!-- done hiding -->
</SCRIPT>

</head>
<body>

<!-- sample -->
<FORM>
<INPUT TYPE="BUTTON" VALUE="Открыть Pop-Up" onClick="popHelp()">
</FORM>
</body>