Crashing computers and slow internet cause 92% according to a recent survey reported by the BBC http://www.bbc.co.uk/news/business-38125619
Many of the problems caused by crashing and slow running computers can be simply down to the amount of dust accumulating around air inlets, cooling fans and processors. The dust acts as a "duvet" around the processor causing it to run above the optimum heat range and, due to the control settings, the machine will eventually freeze, run slow or shut down.
Regular internal valeting of computers is vital to maintain its "health" and allows workers to remove this distraction from their working life. There is not much we can do, however, about the office moaners and the people eating at their desks!!