Much like your home or your car, if you can open the locks to get inside so can someone else. When it comes to computer security, if ANYONE has legitimate access, then there is a way for someone to break into the system or information to be stolen. The only guaranteed 100% safe system is one that has had all of its components and all of the data media melted down until all that is left is a puddle of smoldering liquid metal. Realistically all we can do is try to make it very, very difficult for the people trying to break the security. The trick, however, is finding the balance between security and keeping the system useable by legitimate users. - TRoach

Recently several DSL and Cable providers have been giving customers wireless routers as part of the sign up package. While the router's firewall does provide a lot of protection from unwanted internet access, if one does not secure the wireless side of the router you are opening your computers & network to anyone passing by with a wireless system. The people passing by may not be intentionally trying to cause you harm nor even be aware they are attaching to your router. However, if their computer is infected with various viruses/worms your computers are open to attack, will be attacked and could be infected by the system of someone simply showing their laptop to your next door neighbor.

If you have a wireless router, always secure it by setting the SSID to a unique name and setting the WEP key(s) on both your router and any wireless computers you have. Then one extra level of security I like to do turn off the SSID broadcast on the router (when that option is available). Doing all this helps make your wireless router very difficult to access without your knowledge and consent.

Of course, if you do not have any wireless systems connecting to your router, I recommend simply disabling the wireless side of the router until you have a need for it.

Forty-five percent of the executable files downloaded through Kazaa, one of the most popular file-sharing programs, contain malicious code like viruses and Trojan horses, according to a new study.

Out of 4,778 files downloaded in one month, Bruce Hughes, director of malicious-code research at security firm TruSecure, found that nearly half of them contained various types of nefarious code.

Data Security: Files containing your personal and confidential information could inadvertently be uploaded for distribution on the Internet. Another user could use that information to commit identity theft, or to embarrass you.
SpyWare Files downloaded from the Internet using P2P software may contain spyware that can track your online activity, control your computer, or harm its operation.
Viruses Files downloaded from the Internet using P2P software may carry computer viruses, worms, or Trojans that can damage your computer or cause other problems.

There are many ways this could happen. However one of the most common is when one downloads and installs software from questionable sites.

Unfortunately there are many sites which in their terms of service fine print indicate that if you download and use their software you are granting permission for them to include ad-ware, tracking, and even auto installation programs. Since the programs are installed by someone manually accepting ALL installation requirements this often allows the spyware/adware programs to by-pass any anti-virus and/or anti-spyware software running on the system.

Thus, the computer gets infected even with up-to-date anti-virus / anti-spyware software.

Often harmless and even helpful looking software downloads can be the cause of all the annoying popups that people are getting on their system.

Click Here to see if the software (or the company name of the software) you are thinking about downloading is listed as having spyware BEFORE you install.If it is listed, you will probably want to rethink your choice and NOT install the code. Not installing will likely save you a lot of grief later.

The site has a good list of known spyware and adware program sites which should be avoided. Though it is impossible to know about and list every site/group.