Hello,
What procedure had you used to uninstall 10.0? Had you used Add/Remove programes? After deleting the Veritas keys from the Registry, had you rebooted the server before trying the reinstall? Do the Backup exec services exist on the server? Also check for traces of the Veritas folder in Program Files. Are there any errors in the event logs?
You can also post your query on the Symantec Email support that handles the Install cases.
Do elaborate with details to the above questions.
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Regards.