Things viruses can damage.

  1. Targeting systems, impose an attack penalty over time.
  2. Memory. Penalties to knowledge checks. If you DM's really cruel loss of feats.
  3. Motivators. Dexterity penalties. Movement speed penalties.
  4. Vision. Spot check penalites. If DM is really cruel illusion effects.
  5. Logic circuits. Confusion effect, paralysis effect, etc.
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.

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.