Liz: if L is the only letter that won't type, try prizing the key off the keyboard (gently!) as there may be something stuck under it. (I had that problem recently and found a large piece of Bombay Mix...) You should then be able to press it (gently!) back into place. If there is more than one letter that won't type, and they're all grouped in the same area, then you may have a defective keyboard.
Re antivirus: many people here, including me, recommend Avast.