Yes, 'down'-grade to Windows XP. I'm not kidding, if you could see my face you would know how serious I am. Try again when Vista has matured a little more.
In the event you're stuck with Vista, the problem is likely related to some permissions issue. You can try changing the 'Compatibility Mode' setting to 'Windows XP SP2'. If that doesn't work, I suggest re-installing Pegasus into a location under your user-space (like 'Documents'), and move any/all email files/folders to that space as well. If that doesn't work...see the first suggestion.