The problem is that the difficult rate increases based on the hashing power put into the network, and the reward for mining is cut in half as more coins are mined until there are none left.

http://www.bitcoindifficulty.com/

You'd need a LOT of regular PCs to match even one simple asic. A butterfly lab's jalapeno does ~5 gigahash/sec. You'd have to infest 500 decent PCs to achieve that, and then wait about a month for about a tenth of a bitcoin.

So to get 1 btc in a month, you'd need to infect 5k PCs without them noticing that "damn, my machine is really pegged" and "golly, this laptop is awfully hot and my battery dies really fast...".

It has happened, but realistically you could just buy a miner and be done with it. https://krebsonsecurity.com/2013/07/botcoin-bitcoin-mining-b...

They were able to get ~240 machines, at a time when CPU mining was making them ~$350/day. Decent, but worth all the time to write the software and distribute it? Eh, probably not.