I really suggest you don't try to waste resources of your machines and electricity. The difficulty today is VERY HIGH, and you won't make any substantial amount of BTCs with your hardware. Enter you MHs/s rate to any calc on the net, like this one:
http://www.bitcoinx.com/profit/ and you'll see that you won't get much.
Instead I suggest you to get a 10$ and buy BTC on MtGox or with any other payment source. You'll thank me in 6 month
If you'd like to mine anyway (may be you suggest that fun and entertaining, like me
you should take a look at alternative cryptocurrencies.
There are several currencies that are designed for CPU mining:
* Protoshares:
https://cryptocointalk.com/topic/1492-protoshares-pts-information/* Primecoin:
https://bitcointalk.org/index.php?topic=251850.0These currencies may or may not increase their value over time, but if they will that may be a good investment of CPU time.