How can I play 1080p movies with windows 7?

OK, so I am running my computer with an AMD Athlon 64 X2 Dual Core Processor 5200+ @ 2.7GHz (overclocked to 3GHz) with an 8600ms graphics card. Now when I was using Windows Vista x86, I was able to run HD movies with bitrates up to around 16,000kbps. Recently I upgraded operating systems to Windows 7 RC1, am using the same codecs and players, and I cannot play movies with a bitrate over around 5,000kbps. What can I do to make everything go back to normal OTHER than downgrading to Vista or XP?