If he added a virus? Well, it'd likely be in his coding. If I ever suspected him of something, I could decompile GT and check it out.
But a clever man would break up such code to make it harder to spot. Or even better only have code that would alter the code of someting else to make it malware.
Some of the best viruses out there use this sort of thing lucky vary few people have the time or dedication to make something like that. But imposable to track to source viruses can be made.
Hopefuly if Greg ever goes down that path (and he might be crazy but I dont hink hes that crazy) it will be like his redstone logic and end up poorly excuted.