用iframe吧
var a = showModalDialog("a.asp");
-------------
a.asp
///
<form action=a.asp target='abc'>
...
</form>
<iframe name="abc" width=0 height=0>
var a = showModalDialog("a.asp");
-------------
a.asp
///
<form action=a.asp target='abc'>
...
</form>
<iframe name="abc" width=0 height=0>
ie5.0不支持