Automated Popup When Cell exceeds certain Value

Hi,

Can anyone help to provide a method to do the following:

On opening a workbook, if a particular value in a defined cell in a particular worksheet exceeds a certain value, Excel automatically generates a popup window informing the user of such.

To put it into context, I store a lot of data in workbooks for clients, split into different worksheets in each workbook (1 workbook per client). Inside is an account summary page and if their account is overdue/underpaid (i.e. value is above 0) l would like for Excel to notify me with a popup every time I open that client's workbook until the account is clear.

I've had a look around, but I've not been able to compile anything that works.

Many thanks in advance for anyone who can help!