registry problem in windows 98se

On a network of 13 pc’s, 6 of them have been showing a very disturbing message: “windows has encountered a registry-problem and needs to restart”
At first I thought it to be due to wrong software installation and therefor limited to one pc. But this is slowly and surely spreading around.
I am thinking of a possible virus, but neither Norton or Mcafee programs have found anything so far, but trojan horses in temporary internet files. Complete reinstallition helps, but i also need preventive measures and answers to what might be the cause.
Anybody have a clue?