hi, (im using a Dell laptop, with Windows 7, x64)

Yesterday i used my new Arduino UNO for the first time. I've downloaded the Arduino Software on the official website, and unzipped the folder. I plugged in the USB cable with my Arduino UNO board. I opened the Arduino.exe and i opened the Blink example. I uploaded it and it seems it was successfull. The LED was blinking on/off. But after that, i've changed the code in the Blink example (i would let blink 2 leds) and tried to upload it. Then i get the error: Serial port 'COM3' can't be found. I looked in the menu at extra--> serial port. This is unclickable and grey.

I searched on the Arduino site (and forum and Google) for errors and troubleshooting but i can't get it working. But i know, i forgot to installate the driver for the Arduino UNO. So i went to the Configuration Window of my laptop, choose Hardware & Sounds --> Device Manager. In the list with devices i tried to find the Ports (COM & LPT), but it was in my list. I neither found 'Unknown devices' in the list. I enabled in the Menu --> View --> View hidden devices. It doesnt work. After that I tried in the menu 'Add older hardware'. There i chose 'Looking for hardware and installate automatically. In the following list i clicked on Ports (COM & LPT). In that list i see Arduino LLC (www.arduino.cc). I clicked on Browse and chose the arduino-1.0.3/drivers/Arduino UNO.inf . In the following list i clicked on Arduino UNO and then on Next.

The installation began and then came the following message: The next hardware is installed: Arduino UNO. The software for this device is installated, but it seems it does not correctly work. This device does not start (Code10).

I have tried everything. I tried it on another computer + Laptop (both with Windows 7), and nothing happened. It seems the Arduino (or usb cable) is not recognized. The LED on my Arduino is still working and the ON light is green. An orange light (L) is ON/OFF/ON/OFF.. I've pressed the RESET button on the Arduino but nothing helps me...

Please, can somebody help me? Thanks in advance. (Sorry for my bad English)