I saw the thread title and also thought USERS.
I'll have to say viruses/spyware/adware because you asked for the most common problem.
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JohnThePilot: Cenedl heb iaith, cenedl heb galon (a nation without a language is a nation without a heart)
John Butler Trio: Go take a step outside, see what's shakin' in the real world - Good Excuse
Paul Kelly: Just add a dollop of tomato sauce for sweetness and that extra tang - How to Make Gravy