The difference between viruses and worms is that viruses need the user to spread them into the machine or send them to another computer. The dangerous function of worm is that it replicates itself without the user even running the setup file. It goes on infecting programs and softwares as well as the files of the operating system, taking up all the free space in the processor making it even slower to perform usual tasks.
The three main sources through which a computer can be infected of worm are as follows:
1.The main source these days is through an internet connection. People have broadband connections of internet and are online all the time browsing through different websites that makes the possibility of getting infected by a worm even higher.
2.The second reason is through email attachments. A person might send you a legitimate file but if the computer of that person is already infected by a worm than chances are that file is also carrying those worms. Once you download and open that file BAM! You are too infected by a worm.
3.Another way of getting infected of worm is through file sharing, peer-to-peer or instant messaging. A file is sent to you by a person you know very well. That person may not even have an intention of infecting your computer but his or her computer might be infected which will bring that worm into your computer as well once you accept that file.
Now the functions of worm are:
1.Uses a compromised machine to spread through instant messaging, mails, sharing etc.
2.It discloses private or sensitive information to the hacker or displays it all over the internet.
3.Changes your settings, wallpapers etc
4.Deletes files and folders of your hard drive without the administrator knowing about it.
5.Causes software instability making the software showing errors whenever opened, hanging of software or closing down without any reason.
6.Your computer becomes really slow making processing really hard.
A known example of a virus, Trojan as well as a worm is the "Love bug". It is a virus for the reason that once executed; it started infecting files on your computer hard drive turning them into new Trojans. It's a Trojan because it came hidden as a 'Love Letter' when really it was carrying a malicious program. It's a worm because it publicized itself by sending itself out to everyone listed in your contact list or IRC client.
Now if you feel you have been infected by a worm. The first thing you probably want to do is to install a good anti virus software with a spyware/malware removal software. Keep the software running at all times if you possess a broadband connection. Keep your software up-to-date to protect your computer from old as well as new worms, viruses etc. The names of some of the best anti-spyware softwares are Microsoft AntiSpyware Beta, Ad-Aware SE, Spyware Doctor, eTrust PestPatrol or SpyHunter. You can download one of this software which is free while some offer free trial version of the software from the internet and then purchase it later on if you find the right one for your machine.