If the installer detects there are pending uninstall actions it will ask to restart the computer. On some systems restarting does not remove this message, whilst the installer has an option to continue with the install, some people would like to know what is stuck on their system.
If you open registry editor (press start button >> type regedit)
Browse to:
HKEY_LOCAL_MACHINE\SYSTEM\\CurrentControlSet\\Cont rol\\Session Manager
And look at the item 'PendingFileRenameOperations'