Probably an avi or a driver issue, like Trotter said. About drivers, since you are using an ATi card, I would recommend staying away from the Catalyst drivers. I personally have not had great experiences with them. However, I personally think that the Omega Drivers are great.
-AMD Athlon 64 3200+ Winchestor
-MSI K8N Neo4-F
-Sapphire Radeon X850 XT
-2x 1024 Corsair XMS Pro
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