It is very likely that in a forum such as this some members download video files a great deal. There is a nasty programme that one can pick up during these operations called QuestBrowse and the programme comes in many varieties and will carry an unwanted programme Adware. It also opens up your PC to outside eyes. It is worthwhile doing a search for this programme or check the programme files. QuestBrowse is difficult to delete as it is in contact with the internet on startup hence always active. It is necessary to load in safe mode to delete the programme in the normal way then check via regedit the software keys and remove any reference to the malware.
I am not a PC expert but just spent an hour or two removing said programme.