Why can't I insert a DVD into my Mac Mini? I'm trying, without success, to insert a DVD inside my Mac mini DVD drive. Let me say that actually the computer is without the upper case and the HD drive has been taken from a Windows laptop (it is NTFS formatted and there's Windows XP installed on it).

I'm tryng to insert Install DVD to format and reinstall a proper version of Mac OS X upon new hard drive, but I can't.
Sounds like your optical drive is faulty, immediate ejection of a disk is typical of an optical drive failure.

In the current state your Mac mini is trying to boot to a Macintosh volume.  So, after pressing the power button press and hold the Option key until a mouse cursor appears and your Windows XP partition should appear.  Using either arrow keys and the Enter key, or the mouse, you can select the Windows XP partition and boot to that.

In terms of getting your optical drive fixed, I'd recommend buying an external USB optical drive, it'd be much cheaper than repairing your optical drive through the Apple channels.