First of all, computers do not have harmful radiation to human body. At the same time, there is no evidence that cactus can prevent radiation.
Like most household appliances, computer radiation belongs to [non-ionizing radiation], which is not harmful to the body and does not need to be prevented. What affects the body is [ionizing radiation]. The ionizing radiation that can be contacted in daily life is generally medical X-ray examination. However, occasionally taking a film and doing a CT will not affect the body.
Only by receiving many, many times of radiation within a certain period of time and reaching a certain dose can health be affected.