Electromagnetic interference (EMI) can be a problem if a system is close to a radio or television transmitter, or perhaps near some high-energy source of radiation such as a furnace or heavy electric motor. This type of energy can cause problems with the power supply, and these can be alleviated by surge protectors and UPS devices, which reduce electrical noise and voltage fluctuations.
You should consider an alternative site or some sort of metal shield if the effect of this is severe. Keep the system away from power lines, transmitter aerials and take particular care in industrial environments.
In the case of a PC, current legislation dictates that the PC must be able to withstand certain levels of EMI and operate as normal. The PC itself also must not produce excessive levels of EMI that could interfere with other electronic components. EMI is a serious issue these days and is the subject of increasing amounts of legislation. Companies have to carry out electromagnetic compatibility testing (EMC testing) to ensure that they meet current guidelines, etc. EMC testing should prevent dangerous situations occurring where critical electronic systems, such as life support units or aeroplane flight control systems, could be affected by other electronic items.
You will often find a section in the documentation provided with your system or hardware that details the EMC testing and compliance details for your hardware.