Wow!! It appears to have changed my Wallpaper with the main title/part reading "Warning Your're in Danger! Your computer is infected with spyware!"

Notice the spelling error Your're very suss indeed. It has applied another Security Center icon in the system control panel and appears to have installed some virus scanner called System Security Version 4.51. The new red shield icon continuously displays a message saying to install the last update of windows security software...

What scanner should i use immediately?! I can if required perform a reformat im not fussed but this is really freaky. Any help at all would be awesome!!