U tried to play .avi and u get msg cant delete file.
OK, file is linked to some process like the player you tried to play(or u did something else that links to that file ) it and it wont delete .
u will see an icon in notification area(next to your clock) so u know it is running and than try to delete.U get again same windows msg,click OK unlocker will pop up and u select delete file.(it will kill the process that is holding that delete.) Done.DigiFixIt
already pointed out for u.