I'm a good deal more worried about what the current implementation of the SETI at Home software might be up to, since it is set up to allow multiple distributed computing tasks, not all of them related to SETI, and you pretty much have to accept that they're benign without much evidence. The old version turned out to be responsible for the apparent failure of the serial ports on my PC, it was stealing interrupts they needed, took me forever to figure it out. Fortunately the new one dosn't want to run on my PC anyway, so I am now out of the SETI business.

As for alien signals taking over our computers over interstellar distances... about as likely as alien diseases being able to attack humans, or human diseases to attack aliens, without physical contact and time to evolve.