Dear JerSand
All the answers you have received will work fine, but I think there's a better way, avoiding some of the drawbacks. These are:
Disabling all the other controls and then enabling them once the user has entered an ID is fine, but will invoive a lot of enabling statements - if you have dozens of fields this would be a pain.
I like the idea of checking for a value before losing the focus (presumably this would go in the 'OnLost Focus' property but you have to be sure exactly which property is best, because the order of events (OnExit, etc.) may mean a bit of experimentation.
Making the field required is definitely a good idea, but your user will only be told when they try to move on - ie when they have already filled everything else in.
My preferred solution would be a dialogue box asking specifically for the ID only. You create it just like any other form, but set it to PopUp and Modal in the properties. You can also size it to come up in a little box of its own, just like an ordinary dialogue box. Give it an OK button and put code (or a macro) on this button's OnClick property so that it can check and give a message if the user has left the field blank.
Setting a from to PopUp means it can come up any size you like even if everything else is maximized, and Modal means the user can't click anywhere else outside it. (So give it a Cancel or Close (X) button as well, otherwise they could be stuck!)
The advantage of this method is that it's absolutely clear to the user that they have to provide and ID.
One other thought - are these IDs going in new records? If they are, and there's nothing special about them except that they provide a unique reference, then why not set the field to AutoNumber and let Access supply it? That way the user can never mistakenly enter a duplicate.
All the best
Paul