Excel Script/ VBA Script

I currently have an excel file and i need some sort of 'watcher' which will run a script if a particular field in the excel file is populated. The script must create a text file and save it when field K3 or M3 = 1. The text file will contain one small line of text extracted from the same excel file. So basically I have an excel file... if field K3=1, run script which extracts text from field P3 if field M3=1, run script which extracts text from field Q3 Then write & save a .txt file to a target folder. **Note: text string can reside in the same excel file in fields P3 & Q3 or if you write in VBA, we can place it there. I must have the flexibility to change the text string. So, I am not sure if this can be done in Excel or VBA.