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:
There were no updates and the definitions showed 6/18/2012. Ran the Quick Scan option and it ran fine. I updated Windows Defender and ran the quick scan and it generated the error I first described. Looks like the updated Windows Defender is a piece of garbage and I wasted hours playing with it.