How to prevent viruses, worms, trojans etc .........?

Well I am talking about viruses, worms and trojans here, there is no straight answer to how can we remain fully protected.
Every computer connected through Internet is prone to various security flaws.

Its surprising to see many people on Internet don't take the basic security measures seriously.

Even, I have found some of my computer engineering fellow mates in my college with antivirus-less PCs.
Many don't update it.

Having an un-updated version of an antivirus is equivalent to not having an antivirus at all. Because we find that most outdated viruses become dormant. So the virus makers release updated versions of their viruses to counter the updated anti viruses.

So what should we do........?

We should first all have a good antivirus installed on system, which should be updated frequently.An antivirus prevents entry of such programs to your PC's and remove them.
Then for all of us using Internet, we require a firewall as well.
A firewall is a great tool to prevent any un-authorized activity and access to your system.
Then there are spy wares, ad wares, mal wares available to increase your protection.To sum up all, many companies provide complete security suite like NORTON, AVG, KAPERSKY, NOD32 etc.

Even after having an antivirus installed on your system Trojans may find entry to your systems. Because this is what trojans are made for. They breach the antivirus wall of your system and gain entry.
But they can be removed by scanning your computer later [which we should do often].