If you uninstalled a previous version, third party defrag apps like O&O
(Diskeeper) do a terrible job of removing and replacing the default
Windows software to its original state.
You didn't mention what OS, but if it is either Win2k or XP, try running
the Windows Defrag Utility. It should fail to start.
If it does, try this:
From a COMMAND PROMPT type,
cd \windows\system32 (or winnt if that is the systemroot directory)
then
regsvr32 dfrgsnap.dll
regsvr32 dfrgui.dll
(to register the Windows Defrag components)
Reboot and launch the Windows Defrag app. If it still fails, go to Windows Explorer
and browse to the Windows \ INF folder (the folder is hidden by default).
Look for a file called dfrg.inf, right-click and select INSTALL.
Launch the Windows Defrag app. It should work.
Now INSTALL the latest version of O & O.