Another thing you can do is have that function return false and then your onclick can just return the function.

Like this:

function popWin(URL, winWidth, winHeight) {
    [...]
    eval("page" + id + ".focus()");
    return false;
}
[...]
<a href="page_to_open.html" onclick="return popWin(this.href, 650, 450);">Open pop-up window</a>