Sorry, I don't think this is necessarily a PC problem. I have followed advice to expand the swap file and set it to a large, fairly fixed size and PD still starts to chew hard disk and will not respond for minutes, even with small files. Only option appears to be a complete unisntall and reinstall of PD9.
I am also running Windows 7 64-bit, though I had the same problem in the 32 bit Win 7. No shadow files. And the remedy is the same. Uninstall - reinstall. Just reinstalling over the old install will not fix it.
And I have found that the original PD release build is more stable than 2504, the current build. Reinstalling may or may not last a couple of 4 minute videos before it's time to reinstall again.
I have also found that if a project is crashed, you better resave the project with a new name, or you will probably end up with a corrupt project and have to start over again.
The unresponsive system issue is more noticeable when mixing clips from different sources. I have a GoPro HD and a Canon HF-100 and mixing the content is essential, as I have the GoPro on a wing mount on the airplane and use the Canon for onsite filming. Still, the systems goes unresponsive even with just one type of file on the timeline.
In my case, the system crashes if I try to adjust the sound volume on more than one clip in sequence. I need to lower the volume on one clip, then do something else, go back and adjust the volume on the next clip. As soon as I try marking one clip, adjust volume, mark the next clip and then as soon as I touch the volume lever, PD will crash.
In general, you need to work slowly. Quick switches between tasks and panes may also generate a crash, though it is very hard to reproduce this issue. I just have a feeling that PD9 is generally very unstable.
Right now, I have spent more than 20 hours on a project that appears to have gone corrupt. I do save new project files along the way and I am hoping that I will not loose more than 2-3 hours work, this time.
All in all, I feel like I am spending more time beta testing than producing videos, which is kind of frustrating.