IMPORTANT:
The version you download must be in the same language as the version you purchased for your activation to work.
Parallels Desktop 4.0 for Mac updates your virtual hard disk format to provide the ability to recover from failures using snapshots. If you want to revert back to an earlier version, you will have to uninstall Parallels Desktop 4.0 for Mac and reinstall the earlier version.
If your job requires Windows, but you love your Mac and OS X, Parallels will truly give you the best of both worlds.