@Erel
I think, he is talking about desktop.
@Put
You can check your Form Title, and if it exists then quit the app.
You can do that using my dzHWdesktop.dll (latest version) posted here
http://www.basic4ppc.com/forum/additional-libraries/384-useful-library.html
Code:
<font face="Courier New"><font color="#0000ff">Sub </font></font>Globals
<font color="#0000ff">End Sub
</font><font color="#0000ff">
Sub </font>App_Start
dzhw.New1
<font color="#0000ff"> If</font> dzhw.GetWindowHandle(<font color="#800000">"null"</font>, <font color="#800000">"Form Title"</font>) > <font color="#800080">0</font> <font color="#0000ff">Then
</font><font color="#0000ff"> AppClose
</font><font color="#0000ff"> End</font> <font color="#0000ff">If
</font> Form1.Show
<font color="#0000ff">End Sub
</font>