It may be helpful to explain to her that the people she had seen were not hurting each other and that adults can chose to engage in sort of a play (just like when you've spoken to her about how women get pregnant) You may let her know that these individuals are actors/models if it was a picture she saw and that they get paid to take their clothes off with other people. When you present this to her, ensure that she understands that adults can do things like that because they see it as fun and that they had used their free will to do so.
You would see how she reacts as her initial anxiety may have been triggered due to not knowing what exactly she was seeing. She may have gotten scared thinking that people were hurt as well as embarrassed or fearful that you'd reprimand her for having seen this on the computer. Your reaction would play a large role in how she interprets the event. She may not get scarred as you continue to reassure her that you're aware that adults do things that children do not, and that you're sorry that she had stumbled onto the computer.
At the same time, you let her know that since she's observed this, you're explaining to her what it was and you'd answer questions if she has any. Then, you may ask her to tell you what she thinks she was seeing. If she feels uncomfortable, you can let her know that she can draw it for you and talk about it later. Point out to her that nudity is not something that people should be ashamed about and that adults do such things in their private homes.
The list below are some books that may help you talk with your child about what she saw. I would only go into as much detail as you think she needs. I would also suggest letting her as question and answering them as honestly as possible. As time goes by all 3 of the books would be helpful. I would also let her take the lead. I would wait and see what she wants to know if anything. You would be surprised at how sometimes leaving things alone is the best idea. If she wants to know more she can take the lead. As far as the computer itself I would invest in a parental internet safety program. There are many out there to choose from but most work in a similar manner. Once you install the program be sure to update often.
I hope this has helped.
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