davegri's solution is a decent one, but I think what was not mentioned is that when the form opens to the mode you decide, the checkbox or option button will have to be set accordingly, otherwise it will be Null. I think its state should match the default. Then when switching between Active/Inactive, you will have to swap the combo box row source in code, and requery the combo box. If there are subforms on this form that rely on any of this, I'd probably do this in the form Open event, not the Load event.
Another method would be to open the form in either Active or Inactive Employee mode and assign the row source to the combo box in the Open event. The downside is that you would have to close the form and reopen to switch modes. Which method you'd employ probably depends on your business needs and current project design.
A third method would be to have another combo labelled for inactive employees. Its row source would be a query that selects inactive employees.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.