This is what I came up with:
In your query add criteria to the AssetNum field [Forms]![frmEnterAssets3]![cboAssetNum]
In your form, make cboAssetNum unbound (more on this in a second).
Change the Row Source of cboAssetNum to SELECT [AssetNum] FROM tblAsset;
In the After Update event for this combo box, put Me.Requery.
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Now about the unbound combo box: If you are using this form for data entry as well, if the combo box were bound to a field, anytime you make a selection it wants to change that field to your selection within the current record rather than changing records. For data entry add another text box or combo box bound to the AssetNum field used only for data entry.
If I am wrong about this bound/unbound combo box situation, somebody please correct me as this is the only way I have been able to make it work with my own DB.