Hi, this is very strange. Lets take a look at a couple of things: First, can you check to see if in the Skype settings if it says start at log in? Second, when the Skype app is in your Dock right click it (Control key + Click), hover over options, and then if there is a check mark next to start at log in click on start at log in (if there isn't a check mark then don't click it.) Third go to System Preferences (Finder → Applications → System Preferences), click on Users & Groups, click on Log In Items and see if Skype is there (if it is click on it and click on the minus button down below.) Now restart your computer and see what happens (if you hold down the power button and wait for the box to pop up make sure that Reopen Windows When Logging Back In is unchecked. Now restart your Mac. If this doesn't work go to the Mac App Store (Finder → Applications → App Store) and search for Bitdefender and download that and preform a full scan of your Mac for virus's (this can take some time.) Restart your computer again and then see if it goes away. If it doesn't go away then go to Finder → Applications → Utilities → Disk Utility. On the left click on Macintosh HD, make sure you are in the First Aid tab, click Repair Disk Permissions, and then click Repair Disk. Apple warns people to back up your Mac before doing this, but I have never had any problems with it. Although, do be careful in Disk Utility because you can delete everything off your Hard Drive if you are not careful. If you follow my directions you will be fine, but if your need further instructions just say so and I will be happy to help. After Disk Utility finishes then you want to restart your computer again. If that doesn't work go to the App Store (Finder → Applications → App Store) go to the purchased tab and download your desired OS X version like Mountain Lion or Mavericks. Again Apple warns you to back up your computer before you do this, but I have never had an issue with it. Continue with reinstalling your desired OS X version. If this answer has helped you please help me by accepting this answer and clicking on the up arrow. Thank you and have a great weekend!