Following is a list of the common internet threats.

• Computer virus are software programs that will try to infect your system and then perform a specific action including trying to use your resources (internet connection, email .etc...) to spread and contaminate other computers. Usually computer viruses are written by geeks looking to make the "news" with their virus. Most often the biggest risk is that they will consume your resources eventually leading to a complete consumption of all your resources.
• Spyware: This type of virus will try to collect private information about you or about your activities and sent over the internet to "collecting servers".
• Adware: This type of software will use your computer to show ads visibly or sometime in the background.
• Malware or "malicious software" this is a newer term that includes:  computer viruses, Trojan horses, spyware and adware.

Dozens of products compete in this sector. Be sure to check out Amazon.com Computer Security for some of the best prices on these products (most orders above $25 get free shipping).

Here are some links to some of the most reputable vendors:

TrendMicro/PC-cillin

Computer Associates

Zone Labs

Some free security software:

• Antivir Personal Edition Classic
• Avast!
• AVG Anti-Virus
• BitDefender
• ClamWin
• HouseCall Onlinescanner