I remember a legal case a few years back when the Pirate Bay guy was arrested in Cambodia for hacking into a government computer. They found hacking tools and stolen data on his machine. He used the legal defense that since his computer was also found to have malware on it that anyone could have put those tools onto his computer and could have used it as a jump-off point for the attack.

It didn't end up working, he was found guilty. But it's an interesting defense, especially if there isn't any other evidence or motive pointing to the defendant.

Given the insecurity of desktop operating system's and the prevalence of malware that any infosec expert could attest to this could become one of those common fallback legal defenses (similar to "my rights against unjustified searches were violated") when there aren't other good options.