I have been on the lookout for a good (and free) software to secure my HD on my laptops. My employer have been using a tool for this, but we did not have licenses for the Vista OS.
But dispair not! There is an opensource alternative even here. And if I might say so, a rather good alternative. I've been using truecrypt for a while now (www.truecrypt.org), and is quite satisfied with it's functionalaty. I use the whole-disk encryption, but not any of the hidden PT or hidden OS features.
As with all encryption software, I do notice a slight reduction in preformance of the laptop. The reduction in preformance is most noticeable during boot up and start up. Once up and running, I hardly notice any performance issues at all.
I can also reccomend to use this software to encrypt removable media such as USB drives.