Security is a process and not an absolute. While your machine may be secure today, tomorrow it may be vulnerable.
Your best defence is knowledge and a healthy dose of scepticism. If an email and/or file does not sound/feel right trust your instincts.
What tools are available
A router is a physical device that joins multiple wired or wireless networks together. Most routers now come with a built in firewall. This is an additional level of protection for your home network. The bonus is that you can now easily share your connection (if your ISP allows this). Just remember that if you go the wireless route you need to enable encryption and please change the default password!
We are using a router here at home and it is quite nice and I don't need to keep an extra computer on to share the connection, just the router (uses much less hydro).
At home here we use Zonelabs ZONEALARM PRO® on our server. If you are setting a home network up using ICS (Internet connection sharing) this is an excellent firewall to use. Zonelabs also makes a free version for home use.
Symantec and McAfee® have great resources on the current and past hoaxes.
SPY-BOT S&D and AD-AWARE are what we use at home for the server and the two remaining Windows machines. Lavasoft also has an free online forum
Again, Symantec and McAfee® are my two favourite anti-virus packages. They provide quick updates on a regular basis and have an excellent library of current trojans/worms/viruses.
Windows® 98 and higher has a tool called 'Windows Update'. This is a quick way to find out what critical patches needs to be installed. Please note that if you are not using either DSL or cable modem you will be online for many hours as these patches many times are very large.