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Modal pop up for check box

I have a modal pop up panel and a check box in that panel. I have set check box autopostback property true. When I check mark the check box the panel disappears.
I want the panel still to be there when check box clicked because I have code for oncheckedchanged. I also have 2 buttons. If any one is clicked, the panel disappears. I tried with OnClientClick="JavaScript: return false;" for the button, but the buton_clicked event does not work for the button.

if checking/unchecking the checkbox changes the UI in the popup panel, use ajax postback to only reload the content of the panel or the updated controls instead of the whole page. Therefore your popup panel will stay there. If it simply do some processing on code behind and does not change UI, consider using webmethod and $.ajax() call.
the same concept goes to your buttons.

Loading the usercontrol asynchronously on click of a button

I have 2 usercontrol, each in different-different updatepanel. Also I have a button outside these 2 updatepanels. On this button click I am trying to call the method in usercontrol.
It doesn't displays the data till button click is not complete, so my both user controls load at the same time. I want to load these user control asynchronously i.e as soon as method in that usercontrol finishes, I want data to be displayed. What should I change to make it work properly.
The method in usercontrol is basically creates dynamic html for the user control.
I am calling the user control method in button click, so it is happening sequentially. UpdatePanel's Unload event occurs at the end, is there a way I can raise this event from button click.

If I understood your question correctly (which isn't that easy), you need some sort of FinishedLoading event in your user controls and have its delegates on your page where you'd set their visibility to true.

Dynamics controls lost on postback

This old chestnut again.
My page is constructed as follows; I have a dropdownlist which is databound on first load. When the user selects a value from this, a postback is performed which then databinds a repeater control.
The ItemTemplate of this repeater control contains a placeholder control. In code behind in the ItemDataBound event of the repeater, I am adding two controls dynamically to this placeholder, a hiddenfield and a checkbox.
When the user clicks the save button, I then want to iterate over all those dynamically created hiddenfields and checkboxes and determine their values. However when the user clicks the save button, those controls no longer exist as shown in the page trace.
I know this is a lifecycle issue and the articles I've seen on this suggest using Init methods to dynamically create your controls but I can't because of the way my page works, e.g. the repeater control only appears and binds after a value is chosen from the dropdownlist.
What do I need to do to maintain the dynamic controls through the postback caused by clicking on the save button?

The problem is when you hit the save button probabily you dont re-bind the repeater and the controls you have added at run time withint the ItemDataBound event are not longer available(because they don't exist anymore)
Why don't you add those control at design time using the Eval function the set up the value of the hidden field?

You just don't create them dynamically just on the on selection change of the drop-down set visibility true or false for the repeater that will solve your problem.on post back you have to again create those control as they are Dynamically created.

Triggering UpdatePanel on a button contained within a UserControl situated on the webpage

I would like an UpdatPanel to update it's content once a button is clicked - this button is contained within a UserControl placed on the webpage.
However, I cannot just simply reference it using the AsyncPostBackTrigger's ControlID property, because the button is not situated directly on the webpage.
Is there any way to reference it to the AsyncPostBackTrigger?

Use RegisterAsyncPostbackControl, e.g. if you have a user control UserControl1 containing Button1 and your script manager is called ScriptManager1:
ScriptManager1.RegisterAsyncPostbackControl(UserControl1.FindControl("Button1"));
Edit:
This will refresh all update panels, except panels that have their property set to conditional. For these panels you need to call UpdatePanel.Update(); in the code for the button.

Does the user control expose the button as a child control? If not can it? If not then you can use FindControl on the user control to find the inner button.

ASP.Net ImageButton stealing Enter key press

I have an ASP.Net page with a usercontrol on it. The submit button for the page is within the usercontrol. There is an ImageButton on the page itself (outside of the usercontrol). Whenever I hit the enter key to submit the form, it always executes the ImageButton click instead of the submit button within the usercontrol.
I'm using a MasterPage for the style, so the form is within that which rules out using the form's DefaultButton property.

Put your controls inside a panel and set that panel's DefaultButton property.

Add focus on page load.
This example might help: http://www.webmasterworld.com/forum91/1611.htm
It sets the focus on submit button, but focus will be lost when you click a text field in the form.