This list is not meant to be all inclusive but rather as a general reference.

Boot Sector Viruses

The earliest viruses would infect floppy disks and hard drives themselves. These made for some of the hardest viruses to clean. This type of virus was capable of erasing an entire hard drive or even destroy a motherboard BIOS making it completely un-bootable and rendering the motherboard useless. These viruses would sometimes lay dormant waiting for a trigger date to pass and then deliver their payload. This type of virus was often difficult to detect until it was too late. These viruses were often passed around from computer to computer like the common cold via a infected floppy disk. The good news is that these types of viruses are almost non-existent anymore. These types of viruses were prevalent in the late 1980's to the very early 1990.

Macro Viruses

The next type of viruses that started to be seen were macro (a macro is a function built into many word processors to automate certain functions) based viruses. Macros are generally useful things to have in a word processor or spread sheet software but can be used for malicious purposes if desired by a virus writer. These viruses infected the macro capability of word processors and later infected spreadsheet programs. As the software for word processing and spread sheets became more sophisticated so did the macro viruses. Only in the last few years have word processors and spreadsheet programs started to warn you before a potentially unsafe macro is run or keep them from running all together. Macro viruses were most common in the late 1990's and are not seen much anymore.

Trojan Horses

A Trojan horse, like the name suggests; poses as one type of software but is really malicious code meant to infect or in some way break or damage your computer. These types of viruses sometimes pose as an update to software you may or may not have installed on your computer.

Email Viruses

Generally, an email virus will come from someone you know and trust or from spam. These emails will sometimes pose as updates from legitimate companies for software that may or may not be installed on your computer. They sometimes come in the form of good-tidings from your friends or family or as replies to messages you never sent. These virus usually take advantage of built in scripting capabilities of email software. These are currently the most common types of viruses.

Worms

Worms tend to be the most difficult type of virus to defend against. These viruses get pushed from an infected computers on the internet or your local network and infect computers that are not protected by an up-to-date antivirus solution or computers that do not have all the critical software updates installed or are not protected by a firewall. Once a computer is infected they usually become very difficult to clean. Cleaning generally requires removing the hard drive and cleaning it on an uninfected computer or running a cleaning software specific to that worm.

Generally, if your antivirus software is up-to-date you do not need to worry about becoming infected by most viruses; there are of course exceptions to this rule when it comes to new undiscovered viruses. The general rule you should follow is to be suspicious of all email, immediately delete any email from persons you do not know especially if the subjects are suspicious. The same is true of subject that seem suspicious from someone you do know. As a rule you should always check to be sure your virus definitions are update before you open a suspicious email.