The possibility is that the box is emitting RF interference when it is decrypting, if the decryption scheme has changed it could be emitting new frequencies.
The probability is that the switched mode power supply went up the creek, not unknown in Sky boxes. The capacitors used in the PSU's are rated at 2000 hours at full rated use, which is
83 days of continuous use. Even if the manufacturer seriously under-rates them to increase the life, they are not going to last more than about 3 or 4 years. Thomson Sky + and Sky+ HD boxes are known to have problems caused by under-rated capacitors, which can fail in less than 2 years. And switched mode PSU's use switching frequencies which fit nicely inside those used by ADSL.