My boss' PC became infected in Jan with a trojan that our corporate anti-virus software could not get rid of, no matter how many times it found it. Our PC support guy wasn't allowed to use any other software for removal.
Second time this happened to him. Also for the second time we ran the Malwarebytes' Anti-Malware program - boom - gone. (free, BTW)
Me: "Hmmm...didn't you have an identical problem at the same time last year, around holiday season?"
Him: "Yep, but a different trojan"
Turns out days before both occurrances, he received a couple of of those online greeting cards from people he knew.
I don't care who sends them to me - aint gonna open them!