From this link:
http://msdn.microsoft.com/en-us/library/bb421308.aspx#OfficeAccess2007SecurityConsiderations%5FEnablingExecutableContentDatabases(After the Home Page opens, click "Making It Simpler" in the Contents listing, then scroll down approx 2 pages more and you will see the following information)
To determine whether a database is trusted
1. Edit or create an Access macro called AutoExec.
This macro, if it exists, always runs first when Access opens.
2. Make sure that the Condition column is visible.
3. In the first line of the Condition column, type Not [currentProject].[istruste
d]
Under Action, select Msgbox, and then type Click on the Options button in the Message Bar above in the Arguments column shown in Figure 3.
4. In a line directly below, type & in the Condition column (to repeat the previous condition).
5. Type Stop Macro in the Action column (or other valid non-executable content).
6. From that line in the AutoExec macro onward, all the other macro commands in AutoExec work only if the user has enabled executable content.