Steady on...
A few years ago I got a virus that popped up adverts for p*rn. It didn't matter who logged on, the virus still ran in the background and popped the ads up.
The Mrs was not best pleased...
But it was a driveby download from a warez site I think (I am not asking for sympathy). Anyhow I had to rebuild the machine in the end.
Up to date AV software, plus Firefox with Noscript is a good start.