Yes, I did uninstall AntiVir to install avast. The files in the other folder (not the System Volume Information) were on my second harddisk in the Folder Windows\System. It’s a bit complicated - this second harddisk is from my old pc (where I used AntiVir) which I put into my new one to copy those files onto my “main” harddisk I use now. It’s possible that I forgot uninstalling AntiVir completely from this disk…

Those files in the System Volume Information are called:
A0047114.exe
A0047115.exe
A0047116.exe
A0047117.dll
and their correspondances (same order as above) are:
NDNuninstall5_48.exe (Folder: C:\WINDOWS)
NDNuninstall5_64.exe (Folder: C:\WINDOWS}
Amcis2.dll (Folder: E:\WINDOWS\SYSTEM)
msipcsv.exe (Folder: E:\WINDOWS\SYSTEM)

Oh dear… the longer I think the more I fear having done only wrong things… :-\

Anyway, I will do as you suggested.