Things viruses can damage.
Viruses could be caught from interface terminals if you're the kind of android who likes to plug into things where he doesn't belong.
- Targeting systems, impose an attack penalty over time.
- Memory. Penalties to knowledge checks. If you DM's really cruel loss of feats.
- Motivators. Dexterity penalties. Movement speed penalties.
- Vision. Spot check penalites. If DM is really cruel illusion effects.
- Logic circuits. Confusion effect, paralysis effect, etc.
If you DM wants androids to be vulnerable to computer viruses he should also have a way for them to recover. This could be an ability of the android to recover damaged data or an ally repairing you.