I wasn't able to recreate this issue on my Vista OS, either. The only problem that I ran across, was that I had to set the droplet's level of User Access to be the same as Photoshop, otherwise Windows Vista would not allow the droplet to run.
Unless you're still using the old action you created on the other machine, and that action is expecting the files to be located in a directory that no longer exists (on the new machine), then I'm not sure what the problem could be.
If you contact me by phone, I can walk you through a clean install of Photo Mechanic.
Support number: +1 503 547 2888
Regards,
Jerry