A virus is a program that attaches itself to another program and spreads from one file to another, causing varying degrees of damage. You may not even notice some viruses ,but malicious ones can erase your data files ,corrupt your applications ,cause your computer to crash and in certain cases ,render your hard drive completely useless.
• The Programs on your system start to load more slowly .This happens because the virus is spreading to other programs or is taking up the system resources.
• Unusual files appear on your hard disc drive or files start to disappear from your system.
• Programs sizes change from their installed versions (Original size)
• Your browser (Internet Explorer, Mozilla Firefox etc), Microsoft applications or other software begins to exhibit unusual operating characteristics.
• The system mysteriously shuts it self down or starts itself up and does a great deal of unanticipated disk activity.
• Your system suddenly does not start up or gives unexpected error messages at start up.
• You mysteriously loose access to your disk drives (Like the flash or hard disk) or to other system resources like printers.
• The virus has changed the settings on the device to make it unusable.