In most cases, but not always, the virus has a destructive purpose. Sometimes they are masterpieces of programming, they pursue the challenge of learning or to demonstrate the security flaws in operating systems or the Internet. Viruses are becoming more powerful and affecting more files, as is the case of the OUTLOOK.PDFWorm, the first worm that comes in a PDF (Portable Document Format), the popular Adobe Acrobat format. Though clearly, the latter can now be regarded as a laboratory virus aimed more to be a proof of concept.
When a virus starts its infection, the code comes into play, depending on your schedule, will be more or less destructive, causing problems to the computer system of the user. Sometimes behave similarly to the biological viruses, although there is a clear difference. Computer viruses are always introduced in the system when the user runs them. So if we have an updated antivirus program and refrain from opening files of suspicious origin, unknown, we can be safe from these, at least in a very high percentage. Biologics are different, we all know that we can look perfect and the simple fact of breathing contaminated, we can get a virus.
However, a Trojan is a program that creates a backdoor (provides the possibility of an unauthorized access) to the hardware of the victim, in such way that it opens a port (or communication channel) through which he can access his system. The trojans are usually found attached to other programs using several utilities specifically designed for it, and hiding his real function by the appearance of a program that apparently works well, as it could be just a screen saver.
Consist of two parts: client and server. The server is the one who allows intruders actually entering our computer, and have even the possibility of sending your IP address to any email address to a mailing list, the ICQ, or IRC channel. The best thing to get rid of them is the common sense: do not accept gifts or executables from strangers.
A Trojan horse (or Trojan) is a seemingly useful, novel, or attractive containing hidden functions that allow, for example, get user privileges (if the program runs), assuming a huge security problem. Trojan horses are typically installed by users inadvertently or by intruders who have gained access without permission to the system through other means.
A lot has been written about how to eradicate worms, Trojans or backdoors on a system. For home users there should be given advice more carefully because they need to understand something substantial in the Windows registry and other subtleties. Is required to maintain a good antivirus, one of the best antivirus if possibly, properly configured and updated.