How will you detect that a computer system is infected by viruses?

We can detect that a computer system is infected by viruses if we notice one or more of these symptoms-

1. The speed of the processor becomes comparatively slow.
2. You may get unusual messages on the screen.
3. The disk drive light may constantly flash.
4. The opening or closing of programs/data files takes more time.
5. The programs may not run. Even if they run, they don't function properly.
6. The size of the infected file increases. Thus, the available memory decreases.
7. The system may frequently restart without the knowledge of the user.
8. The system may not be able to read the contents of CD/DVD/Pen drive.
9. The data files may corrupt or show unusual symbols/characters in place of the actual data.