Well that was a predictable reply bob :). I sort of knew that was coming after I posted.
I was hoping to avoid that, even though I have both online and offine installers archived.
I just did a second restart after Repair, that seems to have changed things a bit. AvastUI not now swamping the CPU. AvastSvc pops up as significant CPU user but not hogging it all the time as AvastUI was proir to repair.
Fingers crossed Repair may have fixed it.
Good luck. How much ram dos this computer have?
Old XP/Linux laptop has 2.5Gb RAM.
I have been keeping an eye on Task Manager and am glad to say the bad behaviour has not returned. Certainly sounds less stressed now.
2.5 gig isn’t bad. Your video is probably using 1/2 gig maybe more. The system will never be a speed demon. 
Not good. AvastUI still stuck at 50% on one user account, but not so on the other user account.
Have done full avastclear uninstall, re-install but remains the same.
Also done a Rootkit scan and quick scan but both found nothing wrong.
So at the moment WinXP can be used by one user but the other.
Very strange.
Did you install the product with admin. rights?
Right click on the installation file and select Run as administrator.
Yes. Removal and Re-Install all from Admin a/c
svchost.exe is using the other 50% CPU.
I seem to have removed scvhost.exe hogging CPU by turning off Microsoft Automatic Update.
That also seems to have helped with AvastUI.exe use of CPU which now rises to a peak but then falls back instead of sitting at a constant high level.
Jury is out whether this is a cure or not.
I have had a similar situation running version 18.8.2356 on XP with Automatic Updates turned off: AvastUI.exe usually uses 50% CPU for a time after booting, but this eventually drops to 0% and mostly remains there. I have had the Automatic Updates turned off for years, and so couldn’t say if AvastUI.exe would remain at 50% should I have them on. I hope you will have a similar experience.
Well after suffering avastsvc.exe (was avastui.exe before) hogging 50% CPU, I got fed up and removed 18.8.2356 from my WinXP rig.
I then did a clean install of 18.5.2342 (build 18.5.3931.0). Annoyingly that took two attempts.
First time, even when I set Update Program = Manual, it somehow managed to update itself back to 18.8 >:(
Second time it correctly asked if I wanted a Security Update, which I correctly rejected to retain 18.5.
Lets see if this version proves to be a bit kinder to my CPU.
Fingers crossed
Hi!
That was my experience, too. But I had to go back to 10.4.2233 as you can see in my sig.
BTW: Not using Windows services could make it difficult to keep your version safe. 
=Snake=
seems avast refuse to run on the latest samsung laptop , it has no support for the snapdragon 680 chip or for an ARMS based machine running Windows 10 , very poor showing by them , atleast i know who not to recommend , one lost user .
Basically these are Mobile chipsets and using the ARM instruction set and not the Windows OS, which this Topic is all about and this forum (outside of the Avast for Android Mobile sub-forum) is all about.
If you are talking about previous MS Windows Surface, I don’t think that Avast supported that either.
See https://www.zdnet.com/article/windows-10-on-arm-what-you-need-to-know-before-you-buy-a-surface-pro-x/.
Also see https://support.microsoft.com/en-gb/help/4521606/windows-10-arm-based-pc
We are working on a version for ARM processors, we aim to introduce it in beta quality by the end of this year, so stay tuned
I ran a search inside the forum, and didn’t find a message thread I’d seen using Google’s Search (I think I know why, now, though). I have a second computer, not the one I’m using to visit the forum today. This one has both Windows 7 and Windows 8.1.
Th old computer is going to be running old games using DOSbox atop of Windows XP, and I wanted to use the Avast Free Offline Installer. The second machine will actually seldom be physically connected to the Internet, nevertheless, I will have exposure when locating and installing the games themselves.
Both copies I have tried of the Offline Installer have been rejected by the Windows XP / SP3 on the old PC, with the report that it isn’t a “valid Win32 file” (or very similar compaint. It might use the term “application”. Interestingly, an AVG offline installer ran, although THAT one now tells me it will have expired in December, 1969, twelve years before any IBM-PC / X86 desktop existed, and seventeen years before Windows 1 existed.
I’d thought I might have preferred to have found one of the message threads already about the Avast error situation, since that’s the AV I’m using on this, my primary system. Perhaps you can direct me to a better choice than this thread is.
P.S. I’m editing this. I looked again at what Google found, and it was mostly threads in here from 2008, with a couple of them only as recent as 2013!
Thank You in Advance,
Kiwi
@the-kiwi. Have you seen this post:
https://forum.avast.com/index.php?topic=220639.msg1533473#msg1533473
Note you maybe also should also download the specific Avast uninstaller (avastclear.exe) for this old XP/Vista version.
Some of us have had cpu load problems running 18.8 on WinXP and have rolled back to 18.5.2342 which seems better in that respect.
18.5 is not abvailable on Avast website, but can be found on filehippo. See link in this post:
https://forum.avast.com/index.php?topic=220639.msg1488900#msg1488900
Thanks, @Rocksteady, however, the version at File Hippo is the ONline one, and the older PC hasn’t yet been attached to my Internet Connection, hasn’t yet run the connection wizard at all. It’s a brand new install a couple of days ago.
I obtained a copy of the Offline Installer from Avast and attempted to run that file. It failed when Windows reported the “Not Valid Win32 Application” error. In case it was a corrupted download, I got a fresh download, but it has failed the same way.
In my initial post to this thread, I had included the “fresh install” phrase, but during an edit, I was adding italics to the two words, and it seemed to work fine, but they are no longer in the current version!
Appreciate your comment, and my apology for wiping out the “fresh install” part!
Kiwi
@ the-kiwi
Try this link for downloading avast 18.5.xxxx https://www.filepuma.com/download/avast_free_antivirus-2/versions/ that should give the full off-line installation file. If you want 18.8.xxxx that too should be available there, though as this topic mentions some have experienced problems with that.
I’m sure this link has been posted in this very large topic or another topic.
EDIT: Set Program updates to Manual after that or it will try t update to 18.8.xxxx which could lead to issues.
@DavidR: Thanks a lot! I have downloaded it to this computer, and copied it to a thumb drive. I’ve let Avast scan the thumb drive nearly every time I plug it in here, in case anything new sneaked aboard, but I really think it will be best if / when both the primary and the older PC are protected!
Kiwi