Time to learn network security

[Spider]

Due to the lethargy within my company being unwilling to invest in any ‘proper’ training, I'm at the stage now where I may as well teach myself anything I need to know. I’d like to know a little more about network security and so called ‘ethical hacking’. How to protect a network, how to do some basic penetration testing, do’s and don’t’s of network security and so on. It will be predominantly a windows environment so I'm guessing there will be plenty of material out there to look through. We have both wired and wireless networks, and have recently started to use the mains based networking solution where the others will not suffice.

Has anyone done any research on this subject and found any particularly useful resources or tutorials?
Any info appreciated on this subject.

Thanks