Like I said before there are ways of doing it see Rogers example here:
http://www.rogersaccesslibrary.com/TableOfContents3.asp
Extract
AutonumberProblem.mdb ( beginner )
This sample illustrates a number of ways to simulate your own Autonumber primary key.
I must warn you that there are problems with using self generated auto numbers. To point you in the right direction, what happens if there is more than one user? This is important if you have or if you are going to have a front end backend arrangement of your database.
I think from memory, the other problem is that if a record is deleted then some systems will fill in the blank record with a new record, the problem here is that unless you delete ALL related tables, your new record becomes associated with the wrong data. (I think I've got that right not 100% though) one solution to this is to not allow any deletions, but have a flag field to flag that a record is deleted, and this excludes it from all of your forms/queries etc.
What I would suggest, is that you keep the auto number that MS Access generates, have it "invisible" behind the scenes and use that to link all of your tables together, then have a separate column for the number that you want your customers to see, possibly your invoice number or whatever, and keep that in numerical order.
This way you preserve the benefits of using the built in access feature, and keep your client happy by having a sequential order number / invoice number.