Google and spyware

At least 4 times in the last couple of weeks, I've gotten a message from
Google (403 Forbidden) to the effect that it cannot process my request
because a virus or spyware is sending automatic requests, and it appears
that my computer might be infected.
To continue, I have to type some letters displayed in a funny way (so
that they can't be read by software).

I've never seen that before. Any idea what's going on?

René