Yes, it is possible. The HCL lists only those products that have been submitted to Microsoft's Windows Quality Online Services for review. If they pass the qualifications necessary, they are deemed fit for Windows XP (or Vista et al).
Since this service is charged for, not all manufacturers and developers choose to have their product certified. This only means that Microsoft has not tested the product for complete compliance with their standards. This is also related to the WHQL certification for drivers.
