Symptoms that may be the result of ordinary Windows functions
A computer virus infection may cause the following problems: 
Note: These problems may also occur because of ordinary Windows functions or problems in Windows that are not caused by a computer virus.

 

Symptoms of Computer virus

 

Windows does not start even though you have not made any system changes or even though you have not installed or removed any programs.
Windows does not start because certain important system files are missing. Additionally, you receive an error message that lists the missing files.
The computer sometimes starts as expected. However, at other times, the computer stops responding before the desktop icons and the taskbar appear.
Symptoms of Computer virus

The computer runs very slowly. Additionally, the computer takes longer than expected to start.
You receive out-of-memory error messages even though the computer has sufficient RAM.
New programs are installed incorrectly.
Windows spontaneously restarts unexpectedly.
Programs that used to run stop responding frequently. Even if you remove and reinstall the programs, the issue continues to occur.
A disk utility such as Scandisk reports multiple serious disk errors.
A partition disappears.
The computer always stops responding when you try to use Microsoft Office products.
You cannot start Windows Task Manager.
Antivirus software indicates that a computer virus is present.
Symptoms of Computer virus

Symptoms of a computer virus

If you suspect or confirm that your computer is infected with a computer virus, obtain the current antivirus software. The following are some primary indicators that a computer may be infected: 

 

The computer runs slower than usual.
The computer stops responding, or it locks up frequently.
The computer crashes, and then it restarts every few minutes.
The computer restarts on its own. Additionally, the computer does not run as usual.
Applications on the computer do not work correctly.
Disks or disk drives are inaccessible.
You cannot print items correctly.
You see unusual error messages.
You see distorted menus and dialog boxes.
There is a double extension on an attachment that you recently opened, such as a .jpg, .vbs, .gif, or .exe. extension.
An antivirus program is disabled for no reason. Additionally, the antivirus program cannot be restarted.
An antivirus program cannot be installed on the computer, or the antivirus program will not run.
New icons appear on the desktop that you did not put there, or the icons are not associated with any recently installed programs.
Strange sounds or music plays from the speakers unexpectedly.
A program disappears from the computer even though you did not intentionally remove the program.
 

Note These are common signs of infection. However, these signs may also be caused by hardware or software problems that have nothing to do with a computer virus. Unless you run the Microsoft Malicious Software Removal Tool, and then you install industry-standard, up-to-date antivirus software on your computer, you cannot be certain whether a computer is infected with a computer virus or not.

http://support.microsoft.com/kb/129972