This problem is addressed in ArcGIS 8.1 Service Pack 1. See Related Information section to download this pack.
When a new XYEvent layer has been created from an OLE DB data source, the 'Spatial select' tool and 'Select by attribute' table window options are disabled.
When you add a new XYEvent layer, a check is undertaken to determine if a unique index exists for the source table. If one exists, it is used as a temporary ObjectID field for the new XYEvent layer. This in turn allows the new layer to support spatial selections and selections by attribute. This functionality is disabled without the ObjectID field.
RDBMS OLE DB connections (such as Oracle and SQL Server) currently don't return any associated tables indices. The resulting XYEvent layer will be classified as a non-ObjectID data source and will not support these types of selections. OLE DB connections to other data sources, such as an Access database, will return table indices. If a unique index exists, it will be used when creating the XYEvent layer.
Make sure a unique index has been created for the source table. This will enable selections for XYEvent layers created from an OLE DB connection to an Access .mdb file.