COM/serial port not available in IDE

I'm using Arduino Uno and Windows7. The port for the arduino board is "COM3" on my system. It is available in windows' system control panel > device manager.

My problem: But in the IDE only COM1 is available in the serial port menue.

Note: Actually COM3 is available, so I'm able to work with it for a while, but after uploading some sketches the menue entry COM3 somehow vanishes, so that only COM1 remains (I can't use of course). I have to reboot in order to continue my work.

I must admit that I disconnected the USB cable and then re-pluged it in (COM3 entry in device manager remains available). Maybe this is the cause?

I think the problem is solved. I was logged in not as admin.