The Pr trial does not support any variant of MPG video for some licensing reason. Why Adobe fails to make this known BEFORE you download it continues to be a mystery to me.
Also; CS4 does not support XP64 OS.
It does not officially support XP64 but I have been using it fine with XP64 for about a year and a half.
Not sure if it has caused any issues. It just gives Adobe an excuse if something is not working correctly. Which fortunately for me I have not had any real problems.
I have read several times in this forum that AMD does not do as well as Intel for video editing
A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0
And even more hardware discussion http://forums.adobe.com/community/premiere/hardware_forum
Including this post about 3 possible builds http://forums.adobe.com/thread/598040?tstart=0
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with the DETAILS asked for in the questions at the end of the troubleshooting link