It's very long ago that I used XP.
First step is to tell us where you bought the Arduino (link please). You might have bought a clone that uses the CH340 chip and you need a driver for it.
Other step:
Open the device manager (just in case: that is not the IDE but part of windows) in Windows XP. Look for the section with something like 'ports' in it (not sure of the exact name). If it's not there, don't worry.
Next connect the Arduino using the USB cable. That section should now show up if it didn't before or it should now show a new com port.
If it does not show a new comport, you're either missing the driver or your board is faulty.