Check out Brian Boston's FAQ:
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7) When I click "Add Printer", nothing happens at all or I receive an
"Operation could not be completed" error message.
In case you have LanTastic installed, navigate to the %windir%\system32
folder and replace printui.dll with mprintui.dll (mprintui.dll is the
original version that the LanTastic installer replaces with its own
version). printui.dll is the system component hosting the Add Printer
Wizard.
If you receive this error and do not have LanTastic installed, one of the
system DLLs that are needed for the Add Printer Wizard to work may be
missing or have been replaced by another file. You will need to restore the
old file:
1. Open the command line
2. Click on Start, then Run and type "cmd" and press enter or click "OK".
3. At the "C:\>" prompt, type "sfc /scannow" to run the system file
protection scanner. This may prompt you for the Windows CD to restore the
original file versions.