The survey, conducted by Vanson Bourne and commissioned by supplier Network Associates, found that two-thirds of respondents were not aware of problems until users alerted them.
Two-thirds also said they experienced significant downtime once a quarter, and more than one third said such problems occurred every month.
A quarter said they did not know how much bandwidth was being used for non-work activities, while almost three quarters said they have had to increase bandwidth and server capacity over the past year.
"IT staff are being forced to work blind and IT directors are taking a major gamble with the health of their networks," said Graham Opie, director at Vanson Bourne.
"It is the same as a surgeon asking a patient what is wrong with them before starting an operation, rather than conducting the appropriate scans and x-rays.
"In the same way, a user only goes to an IT person once the damage has been done and the network is already suffering."