Gave up and did clean installs on two PCs. All was well for 24 hours, but now, on both computers, when I do a Systems Files scan with AVG, I get three "change" warnings, (all on countdown clocks):
C:\WINDOWS\system32\user32.dll
C:\WINDOWS\system32\shell32.dll
C:\WINDOWS\system32\ntoskrnl.dll
However, after that, AVG says 'no virus found'.