I am guessing that that date caused Windows Defender to do some sort of extraordinary scanning that it would not have otherwise done. By that I mean that it probably searched every little item in the computer
I think that is a decent guess, but I don't think that is it - but admittedly, I am working on a couple "guesses" too. I note 1999 is commonly used by BIOS makers as the default date (until the correct date and time is set in the BIOS Setup Menu). Note a totally inaccurate date and time is a common symptom of a weak/failing CMOS battery. The two primary functions of the CMOS battery is to (1) keep the "user changes" to the BIOS firmware default settings "alive" in the CMOS memory and (2) keep the motherboard's RTC (real time clock) "ticking" ("counting" to be more accurate)
when the computer is turned off.
If the motherboard's CMOS battery has never been replaced on this older system, I would probably replace it. Typically they are CR2032 wafer or "coin" batteries, found at nearly every battery counter.
While I agree it "appears" Microsoft Defender (Windows Defender is its old name) did a deep scan, the screen shot says "Quick".
Also I just counted every file on my computer by opening an elevated cmd prompt, moving back to the root on the disk (
cd .. until I got C:\> on the command line) on this system, and entering
dir *.* /s. That lists every file and folder in that root directory and then the /s forces it to list every file in every subfolder on that drive too. In other words, every file on the disk. Even with a fast SSD, it took several minutes to list all the files on my boot drive.
And still I only(?) had 589,368 files on C drive. I did the same on my secondary drive and it only showed 13,485 files. That's 1/3 of the 1.5 million files you first displayed. Checking the other computers here, and all had fewer files than this, my primary computer.
Now why do I show ~600,000 on the disks but Defender only scanned 46,000? That's easy. Security programs, including Microsoft Defender know that only certain type of files are used by the bad guys. These typically are files that can be "run", otherwise known as "executables".
This list is not all inclusive but gives you a pretty good idea of the most common file types used by bad guys to insert their malicious code.
By not scanning every single file on your disk, scans not only take up much less time, they also bog down our systems much less, and (especially for mechanical drives) result in much less wear and tear on the drives. Note since scans are "read" actions (not "write") the wear on SSDs is negligible.
I really hope this is true.
It is. EACH and EVERY file downloaded and saved on our systems ARE scanned on the way in by the real-time scanner. Every time a file is modified and saved to disk, it is scanned. Plus, when you call up any file, including one of those obscure file types, the real-time scanner looks for "suspicious" activity and will halt that activity if something fishy is detected.
Last but not least, regardless your primary scanner of choice, everyone should have a secondary scanner on hand for "on-demand" or supplemental scanning just to make sure we (users and ALWAYS weakest link in security) or our primary scanner didn't let something slip by. In other words, "for peace of mind" and I use and recommend Malwarebytes for that.
FTR, Malwarebytes has never, not once found anything malicious on any of my systems here going back to Windows 7 in 2009 with Microsoft Security Essentials (the W7 version of Windows Defender), through W8 and now with W10 and Microsoft Defender. That's a pretty good track record, if you ask me considering 2 of my computers are regularly used by guests, including several ("it can never happen to me") teenaged grandkids.
The only thing Malwarebytes has ever found on any of my systems are a couple "wanted" PUPs (potentially unwanted programs).
So keep your OS and your security programs current and don't be "click-happy" on unsolicited links, then chill. Odds of getting infected are very very slim.
Is it possible you can still be infected? Of course! It is possible Fort Knox might be robbed too. How? One of the guards opens the front door and invites the bad guys in. Or, a super-duper professional targets it specifically and manages to exploit some "unknown to everyone else" vulnerability.
The difference here is Fort Knox doesn't have backup copies of all the gold. But you, of course, have
multiple current backup copies of
all your data, including at least one copy maintained "off-site"! Right?