"Kelly said he had not heard of a virus on the Lundys' computer until he found it.
Named KAK, the virus' role was to "spread itself and be annoying", he said.
The virus would put a message on the screen at a certain date and time, and if the computer was on at the time it would then shut down.
Kelly said the virus had nothing to do with the disorder of registry files.
It was not surprising that any virus checks had not found the KAK virus, he said." link
I think that the virus turned off the computer, but I am not sure, based upon what I have read so far. On the night of the murder, there would have been no reason for a neighbor to take note of whether or not the lights were on at the Lundy residence (the murder was not discovered until the following day IIUC). Therefore, a person who claimed that the lights were on may have misremembered from another evening. Obviously, a photo of the lights on would be a different matter. It seems unlikely that the killer would have remained there the whole time, but I can't rule it out, either. When and where did Mr. Lundy purchase his liquor? That might flesh out his alibi, unless he brought it with him.