Found this usefull informationalso.
Kinds of Viruses Viruses - Viruses are the threats of the internet. There's a hundred different ways to contract a virus, often times without the user's knowledge. And with faster connections being more frequent and programming getting better, the virus danger factor increases. Viruses often attach to EXE or COM files and they can do a wide range of task from corrupting files to accessing an address book and sending the virus to those people. The "best" viruses are self-updating. Trojans - Trojans are viruses specifically programmed to do a certain task often attached to programs. These programs can be attached to another program so that when the original program is run the trojan virus will be installed. Trojans can let the trojan programmer access certain files on your pc, send your password list to the programmer, and even damage or corrupt files. Worms - Worms are literally a waste of space. These programs take up all your system resources slowing down your pc drastically and even take up your Hard Drive space. They replicate themselves into thousands of other tiny little files that in turn replicate themselves causing an exponential increase. They often reside in the depths of your pc and are extremely difficult to find. The "best" ones often hide in hidden folders in the Windows folder making it that much more difficult to destroy. How Viruses Work Stealth Viruses - A stealth virus is one which hides the modifications it has made in the file or boot record, usually by monitoring the system functions used by programs to read files or physical blocks from storage media, and forging the results of such system functions so that programs which try to read these areas see the original uninfected form of the file instead of the actual infected form. Thus the viral modifications go undetected by anti-viral programs. However, in order to do this, the virus must be resident in memory when the anti-viral program is executed. Polymorphic Viruses - A polymorphic virus is one which produces varied (yet fully operational) copies of itself, in the hope that virus scanners will not be able to detect all instances of the virus. These viruses are hard to detect because of their constant change. Most viruses scanners will detect the original but not the newer versions of the virus all the time. Slow Infectors - A slow infector is a virus that which runs in the memory and infects programs that are modified or created. This is to fool many programs that check for modifications in programs for the virus hides what it has done. Fast Infectors - A fast infector is a virus which, when it is active in memory, infects not only programs which are executed, but even those which are merely opened. The result is that if such a virus is in memory, running a scanner can result in all (or at least many) programs becoming infected all at once. Sparse Infectors - A sparse infector virus will only infect a file occasionally. These will count however many programs it was designed to count then infect and so on. Making it harder to track down the original source of the virus. These are hard to find due to the originally has to be found thru many infected files that could be the original. Boot Sector Infectors - A BSI is a virus that attacks the computer on boot. Sometimes halting the boot procedure all together and/or damaging boot files making the system either unstable and crash on startup. Sometimes they won't start at all. These are some of the worst viruses to get because once infected you are unable to run system virus scans through the OS. Companion Virus - A companion virus modifies a file so that when it is run, it runs a separate program as well. (Many trojans work as this kind of virus) When the original file is run the virus is run instead of the original program. Once the virus is done, which is commonly fast enough to go unnoticed, the original program will start. The user will normally have no clue as to anything was happening they did not know about. Armored Virus - An armored virus will use different things to stop the user from deleting, editing, tracing, and more. These can sometimes be deleted by virus scanners but not always. Virus Protection Firewalls - The truth about firewalls: Flawed software programming. Firewalls monitor what enters your system by examining points of entry. You can program them to determine what your pc accepts and from whom. These work for most viruses, keep in mind "MOST". But that doesn't guarantee that it will protect you from all the other dangers of the net, especially hackers. Cause see hackers can buy, download, rip off, crack, etc., the same program and they can usually get around the programming. Virus scanners - There are two main programs: MacAfee and Norton Anti-Virus. These are good for viruses trying to enter your pc but are completely worthless once one affects your pc. If you don't have some form of anti-virus on your pc and it gets infected, installing an anti-virus is a waste of time because often times the virus has already done what it was set out to do. You should have this installed the second you enter the net. It doesn't hurt to have both installed. BOTS - You won't find these on the shelves and you can't download them off the net. These are the best protection for viruses and security because basically nothing goes through them without your permission. They're often used in the warez underground and have the coolest functions and abilities such as invisible mode. Think of them as guard dogs roaming your pc in search of anything unusually new.
But really, I don't open any now unless I know the sender, and as Bronco says, my McAfee on Hotmail would alert me before I did open a bad one. 
