Your computer shows unusual sluggish, generates strange errors, messages or other odd emissions, or sometimes locked up your system and refuses to function properly. All of these are classic symptoms, but you can not assume that your computer is virus-free just because it seems fine. Some viruses do their work silently. The secret to avoiding viruses is to understand how they work.

A virus is a program that has two functions:
1) proliferate (make more copies of itself)
2) activate (at some signal,count ,date and so on,do something-usually something bad like delete the boot sector)

There are only five typically types of viruses; boot sector , executable, macro, worm, and Trojan - plus a sixth type that is a combination of any of them.

Boot Sector viruses
A boot sector virus changes the code in the master boot record MBR of the hard drive. Once the machine is booted, the viruses reside in memory, attempting to infect the MBRs of other drives by spreading themselves to removable media, connecting to network machines and creating whatever havoc they are designed to do by the programmer.

Executable
An executable virus resides in executable files. Once the infected file run, the virus loads into memory,adding copies of itself to other EXEc that are subsequently run, and again doing whatever evil that the virus was designed to do.

Macro
A macro virus is specially written application macro. These viruses will auto start when the particular application is run and will than attempt to make more copies on themselves - some will even try to find other copies of the same application across a network to propagate.

Worm
A worm is a network-aware virus that spreads through applications such as e-mail and web browsers.These worms propagate by reading e-mail address books and sending copies of themselves to everybody. Most will mask their origin by using false e-mail address as the sender.

Trojan
Trojans are true freestanding programs that do something other than what the person who runs the program things they will do. Trojans are quite sophisticated.