Parallels for Mac  4.0 Changing Windows File Associations for Powerpoint

I have been using Parallels for on an Intel iMac 10.6.2 and I have been creating a PowerPoint 2007 files without incident for months. Then suddenly the file associations seem to have disappeared. The extensions which were usually .pptx suddenly became "pptx_auto_file" after my last save of a PPT. I tired doing Save As, and copying the slides into a brand new presentation in case the file had become corrupt. To no avail. When I save the new file it still becomes a "ppxt_auto_file."

I know that you can right-click on the file and select "Properties" of the file and change the launching application from unknown to PowerPoint 2007 (or "Open With" and browse to select PPT 2007 and always have that file open with that application, etc.) But it appears I must do this with EVERY new pptx file I create, which is irritating.

When I opened a Properties-converted file on another dedicated Windows machine, pptx_auto_file name was magically gone! - even though it remains in Parallels.
I suppose this means I can safely send files to clients, but it still bothers me that I must do this Properties change every time I create a new file.  Anyone know a fix for this?