What is infected?
Everything has computers in it nowadays. There could be any number of effects depending on what is infected, what the virus is supposed to do, and what the desired literary effect is. Are you instilling fear? Is it comic effect? Is there a specific (criminal) purpose?
- Your phone starts spamming advertising at you.
- If it makes a funny noise, DESCRIBE the funny noise. A high-pitched whine above the normal operations. Stuttering. Constant cycling of some functions like motors, or cooling systems, etc.
- A robot spontaneously shuts down and reboots.
- The thermostat goes out of control, displaying a cold temp when it's sweltering.
- Your computer tells you you're wrong and to stop ("Dave. What are you doing, Dave? I can't let you do that.")
- An unknown icon appears on the control display. It seems to run and run, but doesn't do anything. You get the blue screen of death, then it resets and starts up again.
- Voice software switching languages spontaneously, or repeating syllables randomly. The voice switches from female to male.
- The background resets to defaults.
- The device brings up a prompt saying it's infected with a virus. Then one of the above happens, and it quits telling you it's infected.
I doubt any of these things apply to your specific case. So ask yourself, "What is the device, what does it do, what would happen if it malfunctioned, and (most importantly) how does the malfunction advance my story or entertain my reader?"