If you can only use client-side scripting, try this script. its not safe, but its safer than the one you use now...
Code:
<script language="javascript">

function open_page() {
var password;
password=this.document.yourform.yourinputbox.value;
loc=password + '.htm'
location.href=loc;
}
</script>

form name="yourform">
  <input type="password" name="yourinputbox">
  [img]image.jpg[/img]
</form>
if you type "mypassword" into your input-box the script will redirect you to the page mypassword.htm.
You have to know the name of the page to get access to it. I would name it something like 23jkli11.htm.

This script doesnt work well with directory browsing allowed =).

its unsafe, its not something I recommend using for any sensitive information but hey, its better than nothing (almost).