NSA Security Guides
Free government guidelines.
When it comes to computer security, the National Security Agency is the shiznit. Or, at least they were. I think they’ve sort of fallen behind a bit, as government agencies tend to do.
In any case, they have a whole website of government-level guidelines on computer security that you can download for free. So, even if they’re not the cutting edge any more, they’re still good guides and free.