I have had no issues with 18.8.2356 on my old WinXP SP3 machine. It does take a while to startup mind.
I have just been back to read this thread and decided to look at cpu load for spikes due to avast. I could not see any during the brief time I observed Task Manager.
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I hope those WinXP guys suffering with Avast problems find solutions, as problem does not seem to apply to all XP users.
PS I have about 2.5Mb RAM in the XP machine. Not a lot, but not minimal.
Ed
1. You won't have to go looking for it, it will be noticeable as your system will slow, browsing, even my mouse pointer and some other tasks. Then look at the task manage and you would see it, if you aren't getting symptoms (great) no need to go hunting (I didn't until it was an issue).
2. For me the solution was dropping back the the version in my system info below my posts. I have 4GB of RAM and the processor is still quite fast.
This issue for me is no doubt linked to the behaviour shield calling/opening the windows unsecapp.exe function, which would appear to have a load overhead on the main avast service AvastSvc.exe (which has the spike).
What changed in the behaviour shield and why it would appear to call unsecapp.exe (and why it would need to) and how it manifests itself upon the avastsvc.exe CPU load is beyond me. I have mentioned this (as have some others), but so far not a peep from Avast.
Which is a bit disappointing after over 14 years of using avast and being a beta tester for much of that time. All I'm looking for is, if this is the last version supporting XP/Vista that it be pretty bulletproof. Not just for me, I can look after myself but for others too.