I remember reading a tutorial on how to do barcode scanning with a LED/photosensor and a cheap PIC microcontroller. Certainly an atmega168 would be up to that task. However, to save you the headache you might want to pick up a used bar code reader. Here in the US you can get the CueCat scanners cheaply on ebay. The interface is identical to that of a PS/2 keyboard.
http://cgi.ebay.com/NEW-RADIOSHACK-CUE-CAT-BARCODE-SCANNER-PS2-INTERFACE_W0QQitemZ250375591290