many winXP users have issues with avastUI not loading after a reboot, since updates from dec '17 & march '18 and later, if you cannot get v18 working, v10 works on XP sp2+ and is from 2015, but better than nothing, and database updates still work.
official link, by senior avast team member:
Download locations
http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe
the post above is for v8 but the same server link now hosts v10.2.228 instead, which is not the best as no HIPS feature and file exclusions may not work [both work in v10.3+] -but grab it while you still can as likely someone forgot to delete it!
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for the last version of 10 [10.4], the non-official link below was kindly provided by regular forum user bob3160, so is probably legit as he seems like a good bloke, it tested ok on virustotal.com, and signature seems valid, but you should do your own checks [or use above links if unsure]:
direct download:
https://docs.google.com/uc?export=download&confirm=iQ1x&id=0B-vSic0iJgyeVDhCcGZ5dlRFUUEoriginal post & link by bob3160:
https://forum.avast.com/index.php?topic=164936.msg1322691#msg1322691