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Essexboy, Windows 8.1 (Standard and Pro) has not been pulled for workstations. Only Windows 8.1 RT was pulled, and it was due to boot problems with the encrypted boot loader. Only a handful of RT tablets experienced it.
I appears that previous versions of Avast (v8, build 8.0.1496 and lower) aren’t fully compatible. 8.0.1497 and higher should be compatible, including the most recent release of v9.
I had my 8.0.1490 stripped out when I upgraded to Windows 8.1 Pro. I’m currently waiting for Avast to release an update for the business products that will work with 8.1.