I would try a complete remove and reinstall of the DM and 4.5. You have to do the install with an account that has local administrator rights. I'm not sure about Vista but in XP there are several locations you can look for installs from RIM.
c:\program files\Research In Motion
c:\program files\Common Files\Research In Motion
c:\documents and settings\username\application data (this is hidden by default)
c:\documents and settings\username\Blackberry Desktop
If you see any of the above folders you can just delete them.
You'll also need to delete the registry key's too. Click on Start, Run, and type in "regedit" and hit enter.
Look under HKEY_CURRENT_USER\SOFTWARE\RESEARCH IN MOTION. Delete that folder and also HKEY_LOCAL_MACHINE\SOFTWARE\RESEARCH IN MOTION, delete that folder too.
Reboot the machine and rerun the install of Desktop Manager. Good Luck
A Fisherman at heart living in the IT world.