Run Popup without Button

[Threewood]

Hi,
I want to open a Popup if a query result isn't valid.

It's an aspx where you can put data into a db.
When you hit the insert button it checks if there is already an entry with the same name.
If it's so, there should come up the popup with a text.
If there is no entry, it puts the data into the db.

For the popup I'm using javascript.

Now the problem: I know how to open the popup with a buttonclick event.
But how can I open the popup without a buttonclick??

Is Ajax the better way to build a popup?

Thanks!