NIST's Systems Administration Guides
The National Institute for Standards and Technology Computer Security Research
Center's recommendations for securing Windows XP & Vista.
FTC Protecting Personal Information Guide
The Federal Trade Commission provides this site for businesses
as a public service to help educate the public about computer
security.
Wireshark
Formerly known as Ethereal, Wireshark is a free, open source protocol analyzer (packet sniffer).
Command-line Reference
Microsoft TechNet's A-Z documentation on command-line utilities, useful for server administration
from the command line.
SS64.com
A nice, thourough reference for command line utilities for not only Windows, but Linux, Mac OS X & Oracle as well. It fills in some gaps in the TechNet reference linked above.