Okay, great info.
Hopefully you tested your iPhone in the same port.??
Windows is a strange creature, and from what I heard Vista is no different of them.
I'm familar with XP, 2k but not ME, Vista or 7 so you will have to forgive me for that.
The fact the whole board has been replaced is good.
I'm presuming here that it used to be Com4, however plugging it into one of the other usb sockets (some comupters have 4) means Windows will go away and load the drivers automatically, and can give it a different port number.
Because Windows has seen this device before (the ATmega8u2 has an electronic signature which tells windows what sort of device it is), it knows what to do, and where to find the driver previously installed.
Can you get into Device Manager (sorry can't tell how on vista) and in step 4 here http://arduino.cc/en/Guide/Windows
see what is on Port3.
If it is the UNO, and the fact you can see a port3 in tools / serial, use this.
Hope this helps...a bit
See how you go with this, and I'll check on vista before the next suggestion.