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I bought a Free-dos computer with 128gb SSD and 1 TB HDD some time ago, installed Windows 10 on the SSD. After months of usage by many people, I question the security of the computer, I see sudden cpu, gpu and disk usage spikes, though I have Kaspersky and tried malwarebytes, I am still unsure of the computers security, I wanted to give it a frest start yet I have many important files inside the computer that I don't have any external storing place now.

My question is, if I were to completely erase windows 10 and re-install it, would it be a good solution for cleaning up (Format SSD) ? I want to store my files to HDD, I won't format HDD but given that the OS is reinstalled, there is no chance that HDD contains active viruses right ? Even if some application had some virus inside, it would need to be run first ? So I can extract safe files from it ?

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Yes, an HDD without OS can contain active virus. For example, one of NSA implants can install itself into the hard drive firmware. The firmware area is not accessible to ordinary HDD formatting tools, so even if you wipe your hard drive with zeros, it would do nothing to this kind of implant.

Another kind of exiting malware installs itself into your bios, and thus would also survive OS reinstall with full disk wipe.

However, unless you're kind of person a three-letter agency would have definite interest about, both are more of a theoretical threat. What is more likely to happen in case you do have an infection is one of your often-used files (such as documents) is infected, and after you reinstall your OS, and open this document, you'd reinfect the newly installed OS back.

On the other hand, sudden spikes on hardware use alone are hardly sign of malware infection. Especially since you're using an antivirus, which itself uses CPU quite heavily.

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  • Thank you for the information. These bios corrupting viruses, are they removable ?
    – Rockybilly
    Commented Apr 30, 2017 at 15:36
  • It depends. The UEFI "infection", which installs another UEFI loader, is easily removable IF you are able to enter your BIOS setup and IF it allows you to change UEFI settings. There is however malware "BadBIOS" which flashes itself into BIOS - this is much harder to remove, and might require hardware modifications for removal.
    – George Y.
    Commented Apr 30, 2017 at 21:38
  • unless you're kind of person a three-letter agency [...] Not entirely true. There are many instances of (rather sophisticated) attacks against individuals performed by non-government agencies.
    – forest
    Commented Jan 9, 2018 at 2:55
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A HDD without OS can contain a virus that will re-activate when the proper OS is installed and the virus properly accessed. If it's an .exe, it will re-activate when you run it. If it's contained within an encrypted archive, it will stay there until you unpack the archive and you run the .exe or script or other dangerous content.

This is a lot more seen than something abstract that will insert itself into firmware or BIOS. Those are usually situations normal users don't encounter.

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