Hello @Vistapocalypse, I am still running Avast Free 18.8 and also Defender. I edited registry as per Avast Forum (I know it was Avast 12.2.2276 & W7 but gave it a try) and runs OK.
Yes, I did also install the last-ever Server 2008 updates on Windows Vista and have not observed any issues.
Just for my curiosity, why have you chosen not to update to build 6003 + sha2 support ?