Why don't AVC-HD files play smoothly on my computer?

view story

http://askubuntu.com – I have a Sony camera that takes HD video and I am having trouble playing it back on my desktop. My system has a 3.0 GHz processor, 2 GB DDR2 RAM and a PCI-E graphics card with 1 GB DR3. It is an older system with Xp and Ubuntu installed on separate partitions. The HD, GC and RAM have been upgraded. I can't get the avchd video files to play smoothly on XP or Ubuntu. What do I need to be able to play these files? I have been able to play them on my ps3 off a thumb drive. (HowTos)