You can import AE projects into Premiere and update them in real time. As far as I know there is no direct way to export the video from premiere to AE. Just import he video into AE, fiddle with it, and then import your projects into Premiere.
You may try deleting everything and when you re-install look for the Mach program under custom installation and uncheck it so it won’t install.
My bet is that Mach 5 is not SL compatible and is causing things to crash on you. Mach 5 is a music sampler. Here is the companies website where you can get contact info if my advice does not help.
I have a canon AVCHD camera and I can watch my file on the computer without any interlacing issues. I do have to encode the file to a format that will play outside or Premiere, but that is very simple to do. What camera are you using?
Sounds like you have enough Horsepower. A quad core processor with 8 GB or ram should be ok, but HD video does take a lot of processor power. I have an 8 core Mac and it runs it pretty smooth. You might have some kind of bottle neck. You may need to run system monitor while editing and see if any app is using too much juice and shut it down while editing.