Spam in your PM box.
There is a rash of spam bots hitting vbulletin boards over the last week. If you receive a PM from somebody that you don't recognize, don't open it. You won't get any malware or anything, but it may be porn. By Thursday I will have a modification done to prevent this from happening, but until then, simply report the PM and the user will be immediately deleted. Our current prevention methods have stopped over 50 of them over the last week, but two have gotten through.