More great advice. I have had to "recover" machines when the owner forgot their password. Booting off a USB drive has come in handy on many occasions, even on a Mac or two. Even without the password, without encryption your data can be read rather easily booting this way.
But as mentioned, this encryption can slow an average machine down to the point of being unusable. A simple way to store your most confidential information is not to save it on the computer at all. I have several USB drives that I keep confidential data only stored on and only access it on the USB drive.
For the average user, none of this is necessary. Hackers look for something that will make them money, such as credit card numbers, banking info and the such. They really wont care about your documents or pictures or 99% of the stuff on your computer.
That is why major store chains get hacked, they have millions of credit cards stored and that info is worth the effort. If stolen, your single PC would most likely be blanked and reformatted with a new operating system.