Missing Codecs

I can play my music files and the video in Vista Ultimate but the sound with
the video does not play. In Media Player there is a flashing note near the
bottom that says "codecs required". In Media Center I get a message "Can not
play video - one or more Codecs needed". Note - the video plays but not the
sound.
Any help would be appreciated.
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Motherboard: Asus M2N-Plus SLI Vista Edition w/Firewire
CPU: AMD Athlon64x2 Dual Core 5200+,2.6 GHz
Video Card: ATI/AMD Radeon 1650 Pro, PCIe
RAM: 2GB 667Mhz DDR2
Hard drive: 250 GB SATA