Thanks for your timely reply!

I'm using Windows 10 Pro.

I was initially concerned I might have a rootkit because of the sheer number of things flagged by my scan.

Also, sometimes for a space when I try to do something on my system (like clicking to open the Windows Start menu or to close a window) it doesn't happen or takes ages. Admittedly some of this could be due to the age of my system's hardware & ?maybe? it'd work better with more memory (I currently have 4 gigabytes.)

The next thing to concern me is recent but unreproducible : (1) a rectangular part of ?the screen or an open window? flashes ?mostly black &/or white, like highlighted text?, ?showing a window that should not be visible as it's beneath another one? & (2) the system beeps as if there's an error or I tried to do something not possible. Trying to flick between windows, e.g. with the Alt + Tab keys, may sometimes trigger this. Note this is not a monitor issue.

Finally, if the occasional quirks I've listed above continue & they're not due to hardware issues, I am guessing they're possibly a rootkit, as I've been performing a series of full non-rootkit malware scans & clean ups of my normal system (some tests remain for the drives I usually don't have plugged in, like my thumbdrive), using different software (Kaspersky, Malwarebytes, Spybot), but the latest scans have turned up (1) no viruses etc & (2) no spyware with a "Threat" bar rating even half-way, with most flagged items looking pretty innocuous.

I note Kaspersky did detect quite a substantial number of issues in files on one external drive which has (unusually) been plugged in & used alot to do a biannual backup this week, with multiple types of malware reported in some individual files. (All these files were deleted before the latest full scan of my normal system, which was clean.)

I suspect at least some files here were falsely flagged (1) because they did contain code to access systems more deeply, but Kaspersky didn't recognise them as legitimate (e.g. ironically this includes an old version of the ZoneAlarm antimalware program's uninstall exe); (2) since this backup drive is hardly ever used & so is unlikely to get infected; (3) because some or all the files flagged may have been on this drive a long time, during which they were likely subjected to scans, which did not find them suspicious; & (4) since the finding of multiple malware issues in single files seems unusual.