In the case of a botnet you would have to consider whether you go by the number of calculations each machine a bot runs on can perform or whether you consider the number of calculations each bot can perform with the risk of detection remaining insignificant.
Realistically you would have to go with the latter in that case I don't think the computing power of most if not all botnets would account for much.
Granted, its more about network resources than calculations of the bitcoin kind.
According to [1], botnets control bots by the millions, and have the capacity to send billions of spam mails a day.
[1] - http://en.wikipedia.org/wiki/Botnet#Historical_list_of_botne...
edit: grammar.