Hi everybody, I just wanted to put in a plug here for what's probably the best book ever on securing Windows-based networks, namely Protect Your Windows Network by Jesper M. Johansson and Steve Riley. The authors of this terrific book both work at Microsoft and are seasoned security professionals, and both of them maintain blogs that are worth watching as well: see Jesper's Blog and Steve Riley on Security. I've been reading and re-reading portions of this book over the last few months and have blogged various tips and insights I've gleaned from it on my IT Reader blog, so if you don't have time to read it yourself at the moment, you can glean some of the best bits from my blog.
But my advice is, buy the book and read it through from cover to cover. Depending on your background, you may not understand everything in the book but you're overall understanding about how to secure your network will nevertheless increase by leaps and bounds by reading it. And although much of the book is targeted at experienced sysadmins and those who design secure networks, there's also a lot of coverage of management-type security issues such as crafting, implementing, communicating, and enforcing appropriate corporate security policies for your company. So buy two copies, once for your bookshelf and one for your sysadmin!
MVP - Windows Server