The most important thing to do before hooking a new computer to your network is to ensure your system has the latest updates and security software installed to help defend against viruses and other potential threats. Of course you want to make sure to always update the operating system and antivirus software on a regular basis.
~~ Take inventory.
Review all the CDs, DVDs, manuals that came with your computer. Put everything together in one package and put away in a safe place. Make CD backups of all your critical software programs. Write down your Serial numbers, System model numbers, etc.
~~ Record system information.
It is a very good idea to record all your system information regarding software and hardware. This will help in case of a reinstall and give the names of your video card, etc. You can use a program like Belarc Advisor - http://www.belarc.com/free_download.html or you can use MSInfo32 which is already packaged within Windows. Type "msinfo32" in the run line. Bring up System Information and File | Export it.
~~ Verify firewall.
Make sure your Windows firewall is enabled no matter what. At least you should use a program like ZoneAlarm to protect yourself. Even on a local network you can have worm or virus traffic. A firewall on each PC will help protect you from that or other bad traffic.
~~ Install Firefox and Spyware protection.
Get and use a browser that has less exposure to worms and spyware. Additionally add software like Adaware and Spybot to protect from possible spyware and malware.
~~ Get software updates.
Use http://update.microsoft.com to update not only your Operating System but also certain Microsoft programs like Office. Also make sure to set the Automatic update to check daily and install patches when your computer is ON and being used.
~~ Standard User.
Set up and use as your primary login a "standard user" account. Do not stay logged in as Admin. This helps protect you from malicious software.