What is a Computer Virus?
A computer virus is an executable file that is designed to replicate itself and avoid detection. A virus may try to avoid detection by disguising itself as a legitimate program. Viruses are frequently rewritten and adjusted so that they will not be detected. Antivirus programs must be updated continuously to look for new and modified viruses. Computer Virus Symptoms: If you suspect or confirm that your computer is infected with a virus, obtain current antivirus software. When a virus infects your e-mail or other files, it may have the following effects on your computer: The infected file may make copies of itself, which can use all the free space in your hard disk. A copy of the virus infected file may be sent to all the addresses in your e-mail address list. The virus may reformat your disk drive and delete your files and programs. The virus may install hidden programs, such as pirated software, which can be distributed and sold from your computer. The virus may reduce security, which may allow intruders to remotely access your computer or network.
Virus Infection indicators: Your computer always stops responding when you try to use Microsoft Office products. You cannot start Windows Task Manager. Antivirus software indicates a virus is present. You received an e-mail message that has a strange attachment. When you open the attachment, dialog boxes appear or a sudden degradation in system performance occurs. There is a double extension on an attachment that you recently opened, such as .jpg.vbs or .gif.exe. An antivirus program is disabled for no reason and it cannot be restarted. An antivirus program cannot be installed on the computer or it will not run. Strange dialog boxes or message boxes appear on screen. Someone tells you that they have recently received e-mail messages from you containing attached files (especially with .exe, .bat, .scr , .vbs extensions), which you did not send.
New icons appear on the Desktop that you did not place there, or are not associated with any recently installed programs. Strange sounds or music plays from the speakers unexpectedly. A program disappears from the computer, and it was not intentionally uninstalled. A virus infection could also cause the following symptoms, but these symptoms could also be the result of ordinary Windows functions, or problems within Windows that is not caused by a virus. Windows will not start at all, even though you have not made any system changes, or you have not installed or removed any programs. Windows will not start because certain critical system files are missing, and then you receive an error message that lists those files. There is a lot of modem activity. If you have an external modem, you may notice the lights blinking extensively when the modem is not being used. You may be unknowingly supplying pirated software. The computer starts as expected sometimes, but at other times it stops responding before the desktop icons and taskbar appear. The computer runs very slowly and it takes a long time to start. Out-of-memory error messages appear, even though your computer has plenty of RAM. New programs do not install properly. Windows spontaneously restarts unexpectedly.
Programs that used to run now stop responding frequently. If you try to remove and reinstall the software, the issue continues to occur. A disk utility such as Scandisk reports multiple serious disk errors. A partition completely disappears.
Source: http://www.a-number1.biz/virus_symptoms.htm
