A short guide to doing JavaScript pop-up windows properly: specify your pop-up function in the href attribute instead of the onclick attribute. If you must use the onclick attribute, specify "javascript://" in the href attribute, not "#" because that will take you to the top of the page, a behavior which is not intended or appreciated by the Web user.
Preferred**: <a href="foo.html" onclick="window.open(this.href);return false;">
Good: <a href="javascript:PopUp()">link</a>
Good: <a href="javascript://" onclick="PopUp()">link</a>
Bad bad: <a href="#" onclick="PopUp()">link</a>
** Milo writes in with an even better method of doing pop-ups: <a href="foo.html" onclick="window.open(this.href);return false;">. Benefits include non-JS browser compatibility and shift/right click compatibility.