You really can't.

http://www.bitcoinx.com/profit/

At the next difficulty estimate, an i7 with free electricity mining all the time at 8 MH/s would make about 9 US cents in a month. Since the difficulty will probably go up again in less than two weeks, make that less than 2 US cents per week. Since very few people actually have free electricity it's not worth it.

http://eviltechmonkey.com/btc.html

Okay, in the hypothetical world where 1 satoshi is worth some huge amount of USD more than it presently is, if you suck up the electricity cost now (which I believe would be less than $1 USD per day for an i7) you can get a few satoshi that may one day make that up-front cost worth it...

Whether you can actually get a single satoshi from a pool is another story. If the pool uses PPLNS, you might not ever. I think most would object to you for being misleading. If you want bitcoins, buy them, or get an ASIC miner and don't use a sucky-ass CPU or GPU.

@Jach:

I specifically addressed electricity costs in my original comment.

My point was, you can make BTC even with a shitty CPU. Whether it's worth it is a different question.

By the calculations above, you would be far, far better off just buying Bitcoins on an exchange than you would be mining them with non-free electricity.