I'm trying to figure out what causes this, my friend is experiencing it on his Dell (lol).

In his words, "for example yesterday i could plug my ipod into the port and upload music on it and today i tried and it says the device is malfunctioning which is bull shit".  He claims all of the devices work fine on other computers and his malfunctioning ports occur at random intervals.

I don't want to improperly diagnose it, so what do you guys think causes USB ports to malfunction?  Failing motherboard?  Shitty Dell parts?