Security is provided in a LAYERED approach, each layer presenting a bump in the road for attackers to get over.
That said, the USER presents the weak link regardless of how many layers are in place.
Why? Because every time you click on a link to visit a page or open an email attachment,
you have no idea what is actually received by your computer.
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First recommendation for cable and dsl users is to always place a router between your modem and the system.
This creates a NAT (Network Address Translation) LAYER to stop alll direct attacks from the Internet.
Basically, your system can not be probed and unless you add port forwarding, none
of the services on your system can be attacked. The addition of the router also
enables SPI (Stateful Packet Inspection; when available on your router) to drop all
out of protocol sequence packets (to protect your services from buffer overruns and other errors).

A good host firewall is the second layer. The XP/Pro (SP2 or higher) has a default firewall which is (barely)
better than nothing. It only controls inbound traffic which would allow a trojan
keylogger to phone home with your bank user/password
Get a 3rd party firewall and it will control both in/outbound traffic
Comodo and Sunbelt are recommended.

The third layer is access control (ie black and whitelist systems) to ensure you don't
access known infected sites. ActiveX is controlled using Spywareblaster, and
bad websites using a host file

The fourth layer is your A/V product. Make sure you configure it to scan incoming email!

The fifth layer is running day-2-day using an LUA account rather than an admin account. The reference also links to the Vista UAC implementation.

If this userid gets compromised, it will not be a system wide contamination and the system will still be bootable regardless

LASTLY: Stay off the online poker, p2p file sharing and porno sites; they are easily compromised
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[edit]
Next: mac-os-x/topic439.html
Security 101 table of contents
contributed by jobeard
[last edit] May 15, 2009[/edit]

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J. O. Beard; you + tech-101.com => synergism. Secure your system now