getandistr
If by chance you have plugged the board into one of the other USB ports from when you first put it in, it may assign a new Comm port number.
The fact the rx/tx lights flash when first plugged in, tends to suggest it is communicating with the computer.
You haven't tod us that you checked the Serial Ports that are available.
Open up device Manager (under ControlPanel System HardwareTab Device Manager look under Ports) You should see one that says Duemilanove.
I spent some time with another user who was trying to upload to Comm4 because thats what it used to be, when the UNO (in his case) was on Comm3.
Mark