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WPF: Display notification box when my application is minimized

How to implement, using WPF, notifications mechanism similar to what eg. Chrome has?
By which I mean: custom content pop-up, displaying especially when my main application window is minimized.
Things I have tried out so far:
WPF Popup class with StaysOpen property set to true. Seems okay, except that it disappears when I minimize the main window.
Custom template window: well, it's a window, so it appears as another application window in the taskbar, which I'd like to avoid.

Enable events in main windows while modal window is active

In a WPF window I implemented a component that gives an indication of all active errors in the system.
The component is located in the task bar and
If one hovers over the component a tool-tip is shown with summary info
If one clicks on the control, a panel is the activated that is showing a detailed list.
In the application we also use wizards implemented as modal windows.
Now, here's my problem: if a new event occurs in the system during an active wizard, the component in the task bar is updated however I cannot use the hover event to see a summary. Both events (hover and click) are not usable due to the modal dialog that hides the parents message loop.
Is their a way (or what's the best solution/suggestion) to have at least the hover message active while the modal wizard is present.
Thanks.

They don't call it "modal" for nothin' :-)
Humor aside, depending on your exact scenario, here's two options and a third option that's a bit more involved and you probably don't need it but it can handle a wider range of scenarios:
Option 1. It sounds like you have a normal app that does not show its MainWindow and is using only its taskbar presence. If this is the case then you don't need a dialog at all, simply use the MainWindow itself as the "dialog".
Option 2. If you are already using the MainWindow visibly then use a pseudo-modal approach whereby you don't actually use a dialog but all other controls except the control acting as your "dialog" control are hidden/collapsed.
Option 3. Use two separate processes. Basically, this is two separate apps - one is your taskbar component and the other is your main app. Use IPC to link the two - e.g. I have used a Windows pipe for such a thing and it works fine.

drop window in application into the main window like tab

i am working on application with multiple windows, but i want to drop any window in the application into the main window like tab and can drop it back like window.
i have tried to used drop event in C# and MouseDown event in xaml but didn't work
so any idea how to do that?

You can't have a child "Window" within a window.
You can have a child user control.
If you still want to have a child window, it would have to appear as a popup
Or you can do a custom implementation like
http://clipflair.codeplex.com/
http://www.c-sharpcorner.com/uploadfile/mahesh/wpf-child-window/
Updated Answer
For docking window you can use third party tools like
Devexpress
Avalon
Telerik
Or for a custom implementation you can try these examples
http://www.codeproject.com/Articles/140209/Building-a-Docking-Window-Management-Solution-in-W
http://www.codeproject.com/Articles/439873/Simple-Visual-Studio-like-Pane-Resizing-Docking-an
http://www.codeproject.com/Articles/22927/Multiple-Window-Interface-for-WPF
http://www.codeproject.com/Articles/18812/WPF-Docking-Library

Make it unable to work in mainscreen when popup is open?

I am implementing an application, and when you start this application, you have a main screen maximized over the whole screen, but also a popup appears. I want that as long as my pop up isn't closed, the user can't do anything in the main screen behind this popup (like when you have an error message or something)? It is probably some property but can't find it out. Someone who can help me out please? I am working in WPF and C#.

Popups are not not meant for this sort of thing i think. You can just use a normal Window and ShowDialog to get a modal popup. If you don't want the frame and everything you can style it respectively by setting AllowsTransparency to true and WindowStyle to None.

Sending command to showDialogue Form from inactive form

I want to send keys to a show dialogue form from another inactive form.See the picture
Form a is showing dialogue.Behind is another form which has a customized keyboard and numpad.
I simply send keys against these button clicks.
How is it possible that I can send keys from keyboard to show dialogue form.

What you're asking is essentially impossible. Once you understand how modal dialogs work (forms that are shown using the ShowDialog method are modal), you will understand why. A modal dialog is used when you want to force the user to interact only with that dialog. It prevents them from interacting with any other windows in your application by disabling those windows. They become impervious to mouse clicks, don't receive keyboard input, and can't receive the focus. Windows beeps at you and flashes the title bar of the modal dialog when you try, it's non-subtle way of shaking its head and saying "no, no, no".
So what's going on here is that when you show your "Deposits" form as a modal dialog using the ShowDialog method, all of the other windows in your application are disabled. In your particular case, that means the window that contains your on-screen keyboard is disabled, too, and can't receive mouse click events. That's why nothing is happening when you try to click on its "keys" (buttons).
The easiest workaround (as I suggested in a comment) is to show your "Deposits" form as a non-modal dialog using the Show method instead. Unlike a modal dialog, this will not disable other windows in your application, allowing the user to interact with all of them at once. Clicking on another window will set focus to that window and allow it to process input events. But you say this isn't workable for you, because you want the "Deposits" form to disable every control on your main window, but not your on-screen keyboard.
Of course, I lied at the beginning when I said it was "impossible". What I meant is that is that it's very tricky, and will require you to work around the standard Windows interactivity model. A couple of ideas
on how you might go about doing that spring to mind:
You could use the On-Screen Keyboard utility that is included with all recent versions of Windows. Microsoft already provides a program for this purpose. You don't have to build and maintain your own, it already includes all the necessary logic to prevent it from stealing the focus when the user clicks on one of its "keys", and since it isn't part of your program, it won't be disabled when you show forms as modal using the ShowDialog method. To check it out, go to Start -> Run and type osk.
For example, in Windows 7 it looks something like this:
If you insist on using your own, custom-designed on-screen keyboard, you will have to show it as a child window of your modal dialog. That is, your application starts with its main form, per usual. Then, when you show the "Deposits" form as a modal dialog using the ShowDialog method, the main form gets disabled. From the "Deposits" form, you can then show the on-screen keyboard form using the non-modal Show method. The main form is still disabled, because it's showing a modal dialog (the "Deposits" form). But the "Deposits" form is not disabled, because it's showing a non-modal dialog (your on-screen keyboard).