These are usually anti-spyware or security software applications that use various forms of deception and/or unethical means or show a history of negligent false positives to goad the end user to make a purchase.
In some cases these applications maybe downloaded with some form of unwanted software at which point the rogue application is offered to the customer as a way to remove the unwanted software.
Security Alert Scanner scans computers and reports detections of malware that is not on the computer.
Security Alert Scanner spreads through Skype spam and possibly other methods.
Skype Spam message (messages may be different):
Software Update ? says: WINDOWS REQUIRES IMMEDIATE ATTENTION
ATTENTION ! Security Center has detected
malware on your computer !
Microsoft Windows Vista
Microsoft Windows NT Server 4.0
Microsoft Windows 2000
Microsoft Windows XP
Microsoft Windows Win98
Microsoft Windows Server 2003