It sounds like it was detecting it before but saying it was in use, now you say it is not even detecting it. It sounds like either the board or the drivers are screwed. I think XP requires drivers even with the UNO so you could try un installing them and then reinstalling them again.
Yup, u are right. It was detecting before but now it wont.
As my comp could not detect the port entirely, I could not uninstall it. I tried adding a new hardware and install the driver, but it does not work either. Not sure if it is the board problem, but the Power light is on, and the Led light will blink upon resetting though.