We recently had to uninstall and reinstall Panda trying to trace down a very strange bug, and when we did the windows Environment Variables were removed, but never re-instated.
I am not sure if it will happen again, or if it were something we did… but if you wanted to look into it, it may be a legitimate issue. Either way we had all the paths on another computer, so we were able to manually put them back in, so all is not lost.