You don't give any information about your system or if
you've been having other problems.  Assuming you have some
version of windows, whenever software is installed, it
updates a 'registry' file.  If the computer is not shut down
properly or if the hard drive is failing, data can be lost.
Apparently, the registry still thinks the scanner is
installed and will not let you add it again.
You can check start\settings\control panel\system\device
manager in your windows system and see if the scanner is
listed.  If so, you can highlight it and click 'remove',
then shut down and restart your system.  You may be able to
reinstall the scanner software now.
The hard drive can be checked by running the scandisk
utility, usually found under programs\accessories.  Use the
option for thorough scan.  This would help identify a
failing drive.

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