Problem with a fake anti- virus popup

Hello,
i need help for like my sisters desktop computer, she accidentally clicked on this thing and the next day when she got on, only her wallpaper would show and this pop up anti-virus thing (which is fake and says "wannabe AVG anti-virus checker" or something) would only show that and NO start-up menu or anything would appear only the wallpaper was seen.

So i was wondering if anyone could help or suggest on what to do to get rid of it as I could delete of the virus if the files were shown but it's practically a blank screen so yeah...

Some of my friends say to reboot it but nothing would show up.... Is there any other way??