There have been numerous cases where people have gotten away with criminal activity because they made the following claims:

The computer was infected - so the malware or the attacker went the the child porn site - not me I stepped away from the computer and left it logged on, so someone else must have went to that website - it wasn't me What do you recommend to refute claims like these? If you had to place a subject behind a keyboard at the time of an attack or crime, how would you accomplish this? What evidence would be useful?