My application was minimized to the task bar and it can't be restored. Windows 7

I have a very interesting problem...

I use Windows 7 and develop on C# using .Net Framework 3.5

When we migrate from WinXP to the Win7 we got a next problem:

Some times when we minimized our application to the task bar it can't be maximized backward. As I notice this happens when dialog window is opened.. (May be in other cases too..). It's a funny, when I clicked at icon in task bar I hear ding, ding... And no expected result, application stay minimized. But when I press Enter my application were maximized. T开发者_如何学Chis can be solution for internal users in our company but not for a customer..

Do you have any ideas?

Thanks.