COM port disappear... Have I broken my board?

I am working on a Arduino Duemilanove board under Windows XP. At the beginning everything went well and the Arduino has a name as COM3. But after several sketches loaded, the IDE suddenly can not find the serial port and COM3 disappeared from the Ports list in Device Manager. The power and TX&RX LED are all on without blinking.

And before this problem happened, the board was becoming very hot near the USB. I am totally new to Arduino and I am afraid I broke this board...Can someone tell me whether the board is broken? Thanks!

What did you have connected to the Arduino? My Arduinos have never even gotten warm.

computer and external power supply :-/

Why both? Which model Arduino do you have?

Arduino Duemilanove ATMEGA328

I just read the introduction book and it said the board will automatically use the plugged power supply. So I tried that :-[

It said that the board would detect which method of powering it was used. I'm pretty sure it didn't say it was OK to use both. I'm prety sure, actually, that it isn't.

Is that mean my board is actually broken? So is there any other thing I should notice when I use the board and don't want to damage it again?

I'm prety sure, actually, that it isn't.

No, that's not the case. Having both voltage sources (USB and external power connector) connected is OK as the automatic voltage switching circuit will use the external power if it's there without any harm.