None.. i just pulled 5 random DVD's off the shelf and all of them work just fine.
I have a desk with a docking station where i pop my laptop in every night. Its in the cubby corner of the bedroom. We rotate the monitor when we want to watch TV or movies. Once or twice a week we watch a purchased DVD and i have never had a problem playing back with WinDVD.
Avid Stamp Collector - I really love 39 Cent Stamps