Workbook you are opening contains macros

I have a large file 2.5MB, when I open the workbook I get the above message. There are no macros I created. There are no add ons running. When I look at the VBA project area there are some private subs. When I open one particular sheet of the workbook my CPU utilization goes up to 99%. excel.exe is using it. When I click onto another sheet the utilization goes back to normal. I am using Office 97, SR2 with windows NT 4.0 SP6. I did have Visio installed but have removed it. What else should I look for?