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:
To verify the manual update worked correctly please check with the Windows Defender (or SCEP) Client UI to see if it reports the latest threat definition version. The latest definition version can also be found in the link above (currently 1.239.797.0).