1) Are you using Windows imaging?
2) Have you successfully restored images to your existing 320GB HDD?
3) What is your new 500GB HDD?
4) Do you connected it alone to the same sata connector the 320GB is normally connected? You could boot to the downloaded Partition Wizard boot CD
and check the new HDD out. Try formatting it NTFS primary with Partition Wizard and repeat the exercise of reimaging.
When you boot from a live boot CD (eg system repair CD) drives letters can be all over the place.