Reposted from talkingtothesky (with permission):
"If you see a post on tumblr that says something along the lines of 'What do you mean, you haven't read J.K.Rowling's 800-word Harry Potter prologue?' and the link is hidden, DON'T CLICK. I ended up with a fake 'Yorkshire Police have locked your computer for illegal activity' virus which made my computer (temporarily, thank god, since I managed to restart in safe mode and system restore) unusable and tried to get me to pay £100. The post I clicked already had 700 notes, so it looked legit, and I was halfway into reading the first two sentences when everything locked down. As evidenced by the 700 notes, it's apparently not affecting everyone, but if there is some way viruses are using that site to target computers, I'd say it's best to steer clear for now, just in case."