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.)
In the London Bitcoin Bank project I remember our average speeds from a pool of 400 people participating at various times was about 40 MHs on average (visual observation). LBTC teams all had GPU's, though some tried to join with CPU's.
This article from Forbes speaks about bitcoin miners in their totality being more powerful than the most powerful super computers combined: http://www.forbes.com/sites/reuvencohen/2013/11/28/global-bitcoin-computing-power-now-256-times-faster-than-top-500-supercomputers-combined/
To your answer practically no. No one computer on its own is going to do anything to bitcoin any time soon. But many computers could, but its not even worth it. Breaking the mechanics of bitcoin is more likely to cause a problem than trying to brute force with computing power.
When one ASIC is equal to thousands of GPU's, forget even CPU's! You are better off trying to break steel with a tooth pick!