Unable to load sketch

Hello everybody,
I've been trying for two days to upload a sketch on my Arduino (Duemilanove) but when I connect it to my PC (Windows 7) and I open the port list in the Arduino IDE there is just one port in it (COM1) even if I fisically have 5 COM ports on my computer and the one I connect the board to is COM5. Obviously, when I hit the upload button in the IDE I get an error message saying that it failed to upload the sketch (in fact the selected port is COM1, since it is the only one displayed, but I connected the board to COM5). The funny thing is that this is not the first time I upload a sketch onto my Arduino. I've done it plenty of times, but it seems that two days ago my ports decided not to work.
Please help me! This is driving me crazy!

P.S.: I've already tried to restart the system a couple of times

there is just one port in it (COM1) even if I fisically have 5 COM ports

You have 5 physical serial ports? That's a most unusual computer.

and the one I connect the board to is COM5.

How do you know this?

Obviously, when I hit the upload button in the IDE I get an error message saying that it failed to upload the sketch (in fact the selected port is COM1, since it is the only one displayed

Displayed where?

I've done it plenty of times, but it seems that two days ago my ports decided not to work.

"All I changed was..." what?

PaulS:
You have 5 physical serial ports? That's a most unusual computer.

It's a desktop PC. I have 4 USB ports on the rear and one on the front side of the case. Oops! Sorry for the misspelling of "physically"!

PaulS:
How do you know this?

When I previously uploaded a sketch (at that time it worked!), in the port list in the Arduino IDE the one selected was COM5, which is the one on the front side of the case of my computer.

PaulS:
Displayed where?

In the Arduino IDE's serial ports list

PaulS:
"All I changed was..." what?

Nothing. Absolutely nothing.

Thank you for replying

It's a desktop PC. I have 4 USB ports on the rear and one on the front side of the case.

USB ports and COM ports are not the same thing.

If you start the IDE without the Arduino plugged in, and select the Serial port option from the menu, there will be a list of ports that the IDE knows something is listening to.

If you close the IDE, plug the Arduino in, and restart the IDE, the list should be different. If not, something is wrong with your Arduino, your cable, or whatever (if anything) you have attached to the Arduino (or something is attached where it shouldn't be, like the hardware serial pins (0 and 1)).

I've tried di plug my Arduino completely naked and THEN start the IDE and still nothing happens (Still just COM1 in the list). The board just turns on and runs the last sketch I uploaded. The cable should be ok because I tried to plug it in with another cable and it does the same thing. Other ideas?

P.S.: Also sorry for my ignorance about COM and USB

I've tried di plug my Arduino completely naked

Put some clothes on!

If the Arduino does not cause a COM port to be created, the FTDI driver is missing, or the FTDI chip is damaged.

I'd try uninstalling the IDE, and reinstalling. Or, follow the troubleshooting instructions, and (try to) (re-)install the FTDI drivers from ftdi.com.

Yeah! Re-installed the drivers! It works! Yesterday I tried to do it but it didn't work... :S

Thank you! Now I can go to put on my pants!