• Programs take longer than usual to load
• Programs begin to crash for no apparent reason
• You Hard Disk is accessed more often
• Files begin disappearing from were you put them
• Devices such as hardware that worked perfectly and now work for no reason

Do not be fooled the littlest thing you do can make these things happen, without you having a virus. There are also more symptoms than just these it just depends on the type of virus you have, I would list all of them, bu the list would be endless.