If you use Skype, never, ever respond to any pop-up message similar to this one:
Several people have reported receiving “fake Flash” ads in Skype which, if triggered, can lead to a ransomware attack.
My recommendation? To be safe, don’t use Skype! There are a dozen or more Skype competitors these days and most of them provide excellent two-way voice and video conversations. Sty away from the products known to have security problems, such as Skype.