Replicate RowHeaderMouseDoubleClick in web form

In the windows form this event opened a sub form using "showdialog"

The sub form had another grid that was populated using the selected value on the grid of the first form eg.

i = PreviousForm.Grid2.CurrentRow.Index

¿I'm trying to replicate this functionality in a web form using asp.net 2.0 with vb 2005

I don't think you can do this, and I can't remember any websites which used a double click instead of a single.
Alternatives you could user are:
Modal popup using the ajax toolkit
On hover panel
Tooltip manipulation.

It really depends what you're wanting to do with this sub grid.

Single click is fine.  The main form is basically a read only screen.  It has drop down boxes and buttons for filtering the grid.
When the user clicks the grid on the main form it brings up a subform with a grid displaying more information for the selected item.  It also has text boxes and buttons for updating and inserting data on the grid.
By default the text boxes also reflect what was clicked on the main form.

personally, I'd build a web user control with a formview for displaying and editing the record (to mimic your subgrid popup), and place this new control inside a template field on the datagrid.  Then, when a record in the parent grid is selected, the formview is displayed in a modal pop up
(requires the ajax toolkit).

I don't have the AJAX toolkit, so was your second solution with or withou it?

I think the AJAX toolkit is easily available, so here are the points.

http://www.asp.net/ajaxlibrary/AjaxControlToolkitSampleSite/

Download, tutorials and advice :)
A lot of people on EE use it too.