The kind of malware described isn't a virus and quite probably was downloaded to the computer when someone "clicked something" giving permission for it to be accepted. Common AV programs simply have no way of objecting if the operator requests a download, or installation of a program. More advanced antispyware and antiphishing programs can give some help via temporary blocking and/or warnings, but a terminally-unaware operator can override the warnings.

The cat is probably hangin' close on the assumption that someone is going to try to name him/her/it as the guilty one.

"The cat did it" is a well known alibi, although it's usually the dog that comes forth with that one. (Sometimes the cat did do it; but you have to know if the dog can be trusted before filing charges.)

John