>>>Can that be done?
>>>How?
>>
>>Click on "Project\[Project] properties\Application\Windows application framework properties\Make single instance application".
>
>Thank you, Michel.
>I'll check that out

I think that's a VB only option. The standard way is to use a mutex. Example here (with a few options on how to handle attempts to open a second instance):
http://sanity-free.org/143/csharp_dotnet_single_instance_application.html