Adobe has changed where you can choose to buy and not subscribe. Go to the following link and click on Buy next to Acrobat XI Pro: http://www.adobe.com/products/catalog/software._sl_id-contentfilter_sl_catalog_sl_software_sl_mostpopular.html. You will be able to change from Subscription to Full or Upgrade. Almir R V Santos.
If you think some malware is preventing the installation of Windows Defender definition updates, you may want to run a scan with Microsoft Safety Scanner or for more stubborn malware, using Windows Defender Offline. See how to update Windows 10 offline. These links are sure to interest you too:
But you can set it so that Windows Defender will check for, download and install updates as soon as they are available, even if you have disabled Automatic Windows Updates. To do so, open the Task Scheduler. You will find it under Administrative Tools in your Control Panel. In the right pane, click on Create Basic Task.