c000021a Fatal System Error

This worries me further. While it just seems like Avast is messing up somehow, the error code it presented exhibits something even worse: The I/O device reported an I/O error. This means a device involved in I/O (very sure it's a storage device) could not perform the operation of taking its contents and putting it into memory because it experienced a particular issue.

I'm sorry but this just incriminates the drive even further. If the hard drive itself is reporting problems, it's time to replace. Now, personally I think it's odd that an instruction is referencing itself (shouldn't really do that), but there's still just something going on here involving disk I/O that's messing up, and the drive itself seems to be to blame.

Okay, I will try taking backups on weekly basis...incase if the HDD fails. I don't think I will buy one soon.

Ok, just making sure you realize if the drive goes down, the system is unusable, so I assume you have a spare system to work on under the condition the drive goes bad.

Yes I do have another machine, thanks for caring about that.
Also, usually, does the HDD die slowly or it will be a sudden one?
After I get the answer, you may mark this case as solved. :)
Thanks!

Every HDD dies differently, some start getting very sluggish, others start corrupting data more and more frequently, and others just plain kick the bucket. It really depends on what actually failed internally on the drive, be it the head motor, the drive motor, or the controller board.

As for marking this as solved, only you can do that, skipper, so mark as solved at your discretion.

I find that if you get this problem it is really quite easy to fix sadly.

Step 1.

Turn Off Computer Fully Then Turn Back On.

Step 2.

As soon as it loads hit which ever button to open startup menu. For mine it is "Esc"

Step 3.

Hit F9 or which ever key loads "Startup Options"

Step 4.

Choose "Load from Ethernet" (or anything like that )(you do not need an Ethernet chord for this)

Step 5.

Chose "Run Windows Normally"

Step 6.

Done!