My DH has become the computer fix-it for our family. Especially for the teens who seem to get viruses on their laptops every week. :p EvilMama would say to have him gibbs-smack them and tell them to stay off porn sites.
A certain grandson of mine managed to infect his parents' desktop PC AND a laptop with such tenacious viruses that they had to be replaced. Yep, porn.
Some game sites can be pretty bad, too.
But I learned to run the factory image recovery and format the hard drive as step one when I had a house guest download first porn, then hundreds of "free games" to the desktop computer that I used to work nights from home (used computer at work for my PT job - used home computer for evening shift, as there was no one else for about three months - they were combining two departments in three months and didn't want to spend the time & effort to train someone else for just 12 weeks).
I had to get the install disks from the IT department to set up my computer for working at home again - but that was safer than using an infected computer with really nasty stuff on it (porn, malware, or whatever she had gotten in the downloads).
I made the mistake of trying to clean it by deleting one photo at a time...after the third or fourth really nasty photo - I grabbed the recovery disk pack and got started on restoring my computer's virgin "fresh from the factory" status - as I had already backed up the photos & most files earlier.
I should have learned to take back up images back then....it would have simplified things - but I had a 10 gb hard drive, a 256 mb flash drive, a 3.5" floppy drive, not enough floppies in the house, and no DVD burner - just a CD player. I'm trying to remember if CDs could be burned at home way back when in 2001....
I did learn to set up a guest account with extremely limited privileges - none of which involved downloading ANYTHING.