So you installed Office 2013 Preview and decided you didn't want to use it and thought you uninstalled it... but like many Office betas, uninstalling it leaves stuff lying around.

Microsoft has an uninstaller for the Click to Run version as well as instructions for anyone who prefers to uninstall Office 2013 themselves. Details can be found at Uninstall Microsoft Office 2013 or Office 365

After uninstalling Office, reboot then do a reinstall of your old version. Because Outlook 2013 moves the Profile keys (now in HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Profiles), uses a more compact *.ost format, and keeps IMAP mail in an *.ost file, the Outlook 2013 profile won't work with older versions of Outlook.

Also note that if you used an IMAP account in Outlook 2013, it now stores calendar and contacts in an offline data store (*.ost). You'll need to export any calendar items or contacts added to the IMAP folders before uninstalling Office 2013 or removing the account from Outlook as the *.ost file can only be opened by the account that created it.

Removing orphaned Control Panel Mail icon

Uninstalling Office 2013 Preview left behind the Control Panel "Mail (Microsoft Outlook 15)" applet. It shows as a broken shortcut with a white paper icon. How do I remove it?

Check the following registry locations for an entry for Office 15, starting with the first keys. As always, you should export the keys before deleting them.

Deleting this key should remove the Control panel Mail applet.

HKEY_CLASSES_ROOT\Wow6432Node\CLSID\{A0D4CD32-5D5D-4f72-BAAA-767A7AD6BAC5}\DefaultIcon
Default value: "C:\\Program Files\\Microsoft Office 15\\root\\Office15\\MLCFG32.CPL,0"

Note: delete the entire key (folder icon, on the left), not just the default value! You can export the key first if you are worried it will mess up your system. Right click on the key and choose Export the right click again and choose Delete.

If not, check these keys and delete the values that point MLCFG32 to the Office 15 path.

Control panel key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Control Panel\Cpls 
Name: mlcfg32.cpl

Control panel key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Control Panel\Extended Properties\{305CA226-D286-468E-B848-2B2E8E697B74}
Value: path to MLCFG32, such as C:\Program Files\Microsoft Office\Office15\MLCFG32.CPL

Control panel key:

HKEY_CURRENT_USER\Control Panel\MMCPL

Clients key:

HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Microsoft Outlook\shell\Properties\command
(Default) value will be something like: rundll32.exe shell32.dll,Control_RunDLL "C:\PROGRA~1\MICROS~1\Office15\MLCFG32.CPL"