Has the Bitcoin network surpassed the total computing capacity of all the general purpose computers in the world? I would estimate there are roughly 2 billion computers in the world, with an average hashrate of 10 MegaHash. That equals to 20,000 TerraHash or about 20 PetaHash. The Bitcoin network is around 20 PetaHash now. Does this mean that the network cannot be attacked using regular computers anymore?
The comparison doesn't make sense. You can't use Bitcoin ASICs to do general computation, and using CPUs to mine Bitcoin is incredibly inefficient.

It also isn't meaningful in terms of security: no one would try to attack Bitcoin by taking over all of the CPUs in the world. Instead, they would fabricate or buy mining ASICs. (Or, more likely, try to hack into the pool servers that miners get their work from.)