Please help!! Laptop infected with a virus called 'my security alert'

I have a small laptop which has recently acquired a new icon called 'my security shield'. It pops up onto my screen intermitently and makes a screaming noise and announces my computer is infected with viruses and I need to download this security alert costing x amount of money. I have tried going into Safe Mode with networking when I reboot my computer to download an anti-virus but am unable to connect to internet in this mode. I have tried downloading various anit-viruses in normal mode but have been unable to do this successfully. It keeps telling me to delete 'my security alert' but when it gives me the options of what I can delete on my laptop I dont know which one it is as none of them sound like 'my security alert'. This may sound a bit confusing but I am very stupid when it comes to computers and have no idea how to get rid of this!! Please help!

Hello.

Download OTL by OldTimer to your Desktop.

• Close all windows and double click OTL.exe
  
• Click Run Scan and let the program run uninterrupted
  
• It will produce two logs for you, one will pop up - OTL.txt, the other will be saved on your Desktop - Extras.txt. Post both logs in this thread.
  
• You may need to use two posts to get it all.