Why can't I insert a DVD into my Mac Mini?
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.