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  A computer virus is an executable file that is designed to replicate and avoid detection once on your system it will spreads from program to program. Viruses can damage data, cause the computer to crash, randomly display pornographic images and various messages. Or they may lie dormant in your system until a given time when they will awake and cause all this trouble, such as at a memorable time or date or when the day and the month add up to make a specific number.

              Most viruses come via e-mail and instant messaging. When they arrive in a e-mail message they appear to be totally innocent messages, such as selling items, pictures, greeting cards even screen savers. Or there is just some silly comment in the subject line

If you think you have a virus, Trojan or worm on your system run your anti-virus program to attempt to find it.

These are just some of the effects an infection can have:

1. A copy of the infected file may send a copy of itself to all your contacts in your address book.

2. Your disk drive may be reformatted.

3. Programs can be hidden on your disk to allow hackers easy access to your system.

4. Erase system files and programs.

These are some virus symptoms.

1. Poor system performance. As the virus will use up the free space on your hard disk.

2. Anti virus program will not run or be installed on to your system.

3. Odd things start to happen. Images appear out of no-were.

4. Music or sounds start playing.

5. Windows will not start.

6. Modem activity. getting connected to the internet.

7. System freezes or shuts down.

8. Windows shuts down and restarts one it's own.

9. Disc utilities report multiple errors.

10. Someone has just received an e-mail with attachments containing multiple extensions from you which you did not send. (.exe, .bat, .scr, .vbs)

11. Windows will not start at all.

12. Windows continually restarts

13. your anti-virus program shuts down and cannot be restarted

14. "Anti-Virus tells you"

Damaging effects 

The most common effect of viruses is an attempt to destroy data on the hard disk. It is worth mentioning that the quality of the virus program and the extent of the damage are not necessarily in line. 
Primitive viruses simply overwrite the contents of files without warning. In this case the file can only be recovered by restoring it from backup. It can be time consuming but if a backup exists it is not the end of the world. 
And if backups are not available then give the responsible person enough time to pack their things and, according to extent of damage, go home or to nearest airport. 

There are more insidious forms of destruction - slow, hardly noticeable changes in data. If a virus which controls disk services has been active for some time it can damage some (if not all) of the backup copies. Sorting which backups have been affected can be laborious or impossible. 
Macro viruses play games with users data files. For example WM/Wazzu puts the word 'wazzu' at randomly selected places in document. If you try a search on Internet pages which contains the word 'wazzu' and are not about Washington University or viruses, you will be surprised how many pages were originaly prepared in Word infected with WM/Wazzu. 

And you can sure imagine what would happen if a macro virus for Excel slightly changed the values of some cells in your XL

Recovering from and preventing a Virus

First thing to do is to scan your computer with your updated anti-virus software, if you do not have on installed trendmicro offers free online scanning.

If a virus is detected remove it, once that is done rerun your anti-virus to check to see if it has all gone.

If the virus has erased system files or it cannot be removed your may need to reformat your hard drive and reinstall Windows

To configure Outlook Express 6 to block access to virus attachments click HERE

Safe and unsafe file extensions

The following list of file name extensions lists types of files identified by Microsoft as potentially containing dangerous programs.

Any file received as an email attachment with any of the above extensions should NEVER be opened even if you know the person that sent the file.

Unfortunately some email programs don't display file extensions in their default configurations, in particular, Outlook Express.

The display of file extensions can be turned on, the method varies slightly depending of the version of Windows, but generally is similar to the following:

1. Open 'My Computer'
2. Find 'Folder Options' (Usually on the Tools menu but possibly on the View menu.)
3. On the View tab, remove the check mark beside 'Hide file extensions for known file types'
4. Click the OK button.

Now you will be able to see all file extensions but the list of dangerous file types is quite long, how do you remember them all?

It may be easier to remember the common safe file types:

Safe File Extensions

If an attachment does not have one of these safe extensions its best not to open the attachment. Be especially suspicious of any file that has a doubled extension (eg. coolpic.gif.exe). Normally files have only one three or four letter extension so a file with more than one extension is probably an attempt to trick you into opening the attachment.

Also note that a file could have a name like www.yahoo.com, it looks like a URL to a web site, but if you check the dangerous extensions list above you will notice that .com in the extension used by MS-DOS applications. This was the trick used by the recent 'My Party' worm.