Macro that works 90% of the time

Hi all,
I am stumped. I am working on a project that has about 50 or 60 worksheets in it. To make it easy to get around, as the first page I decided to add a clickable Table of Contents. This basically consists of 50 text boxes that each have a separate macro assigned. When you click on the text box that says, "Chicago" the macro transports the user to the worksheet called "Chicago" via the following code:

Sub Macro4()
Sheets("Chicago").Select
End Sub

This works great!....90% of the time. This workbook was emailed out to 40 people and out of those 40 people, 4 of them have written back saying that when they click on ANY of the 50 text boxes they get an illegal operation error and Excel shuts down. I have had 2 of them email me back their copies of the workbooks so I could see what is happening and even their copies work fine for me. I am using Excel 2000 but I have tested this in 2000 and 97 and both seem to work fine. I have searched all over the web looking for a possible solution but have yet to find one.

Anyone have any bright ideas?