** Edit: The problem was the Wacom tablet driver conflicting with the Sparkfun Explorer. I uninstalled the tablet driver and it works perfectly.
If I plug my xBee after my computer boots up, it doesn't show up in the serial list. However, if I plug it and then turn on the computer, then I see it in the list, however, if I unplug the xBee and replug again then I can't see it in the list again. Everytime, I need to restart my computer with the xBee plugged in beforehand.
I didn't do any update or nothing changed in my system. Short time a go, no matter when I plugged in my xBee, it was always showing up.
* I connect my xBee through Sparkfun xBee explorer.
What may cause this problem? Any suggestions?