The NSPCC is claiming that not enough is being done to protect children from disturbing content online.
The charity has conducted a survey, which found that 377 of 497 children who took part in an online poll claimed to have been disturbed by internet images.
One child posted a comment on a There4me message board saying: "I've seen violent images I didn't search for. I was freaked out".
Another added that his 8-year-old sister's search for "pictures of animals" generated pornography adverts.
Policy adviser Zoe Hilton responded to the BBC: "Children are just a few clicks away from innocently stumbling across upsetting or even dangerous pictures and films such as adult sex scenes, violent dog fights, people self-harming and children being assaulted".
The NSPCC is calling for computer manufacturers and retailers to do more to help parents get PCs protected.
Hilton added: "High-security parental controls installed in their computers would help shield them".
"Currently computer manufacturers and retailers leave it to parents to find and install software that filters out material unsuitable for children. This can be a complicated process for customers."
The charity is specifically calling for safety software to be installed before machines are sold and also for manufacturers to start building such controls into their products.
She added: "Social networking sites must also put more effort and resources into patrolling their sites for harmful and offensive material and ensure their public complaints systems are clearly marked, easy-to-use and child-friendly".
"We would also recommend they give information on their sites about sources of help and advice, such as Childline, for children who have been affected by what they have seen."