A virus is a software programme capable of reproducing itself and usually capable causing great harm to other programs on a computer; "a true virus cannot spread without human assistance".

A computer virus can cause the following problems:

Render a computer useless- Some viruses can prevent you from being able to use your computer at all. The CIH virus of 1999, named after its Taiwanese creator, Chen Ing Hau, activated on its creator's birthday. It made computers virtually useless by overwriting a critical part of the computer's memory called the basic input output system (BIOS), which handles even the most basic tasks, such as starting the computer. Users had to have a chip in their computer replaced before the computer would function again.

Physical Damage- Other viruses damage specific parts of your computer. One virus, a Trojan virus called WinFixer, coerced users into installing it by telling them there was something wrong with their computers. It was capable of physically damaging the mechanism used to open and close the CD drive, preventing users from inserting or removing discs.

Deletion of files- Some viruses delete your files to cause havoc. One variant of the Mydoom worm, for instance, periodically searched the drives of infected computers for files with extensions like .doc (Microsoft Word documents), .xls (Microsoft Excel spreadsheets) and .jpg (JPG images).

Disabling Functionality- To further spread and to evade removal, some viruses can disable programs that deliver updates and remove viruses. As of April 2009, variants of the Kido worm, also known as Conficker and Downadup, check once per second for running antivirus programs. If an antivirus program is running, it is closed immediately. The worm also disables the Windows Update service and blocks access to antivirus websites, preventing many users from removing the worm from their computers.

But if your lucky (this is rare) it will do nothing, just spread itself without doing any danger whatsoever.

Viruses can be prevented though and here are few  of many ways you can do this:

Install anti-virus software- good ones are AVG, Norton Anti-Virus, Sophos Anti Virus and McAfee Anti Virus

Install a QUALITY firewall- There are many firewalls available, some good, some bad so make sure you get a good one!

Install and anti-spyware software [4]- Like firewalls there are many available, some good and some bad so pick a good one!

Email Attachments- BEWARE OF EMAIL ATTACHMENTS. Don't open them unless you requested the attachment, that is the way some of the worst viruses can be spread easily. Even if it looks like a friends email or an admin's email, don't open it unless you requested it. Don't open files ending in: .exe .vbs .scr .vbe .com .bat .shs .cpl .dll .ocx .pif .drv .lnk .bin .sys .eml .nws from an email.

Keep a regular BACK-UP of important files- You can use floppy, zip disks, writable CD's, tapes, another hard disk, memory sticks etc.

Above I mentioned that a good way to prevent viruses are Anti-Virus Software however there are drawbacks from using these.

• Most good software costs a lot and you still have to pay yearly (sometimes monthly).
• It can instantly slow down your internet connection when you connect to the internet.
• They can claim that things that aren't viruses that you are downloading are actually viruses (Norton Anti-Virus did that Pegasus Mail in the past).
• They effectiveness drops after a while. One magazine found that in 2006 the effectiveness dropped by 40-50% and 20-30% more in 2007.
• Not-so-new software most likely cannot detect new viruses that have been made.
• If a file is damaged by a virus the software usually takes the virus out of the document but does not return the file to its undamaged state.

So there are some drawbacks but there are also many advantages, you can choose to use anti-virus software but you could also use other things like a firewall.