This cannot be done, but a trigger on the field that stores "value" can be set to run a script when the value in the field is changed. The script can exit without doing anything if the value is not less than 0.
If you describe what you want to do here in more detail we might be able to suggest a different option that produces the same result.
I have a field that has special notes in for customers and it is being ignored, so i want to have the following:
when a record is viewd or edited and this field "customer notes" has some content then a big window will pop up showing a really big version of the note and the user will have to close this window after reading it to continue.
I have got the window working, the field will not be entered or changed, but it will have content or it will be blank.
I guess I will run a script trigger on layout load, so that when it opens if field "customernots > 0 then run script "pop up window" ??
Or possibly OnRecordLoad, depending on your interface design.
Be careful on the use of all such layout based script triggers. If you ahve any scripts that navigate to this layout, they will also trip the same trigger.
I just got this working but need to play to make sure it is what i want. thanks for your help!