What can be done with a computer with full access?

Question

Information Security

For example, the computer was visited by some bad people. What could be done for further surveillance? What options are there to get rid of this theoretical surveillance? Will a simple reinstallation of the OS help with formatting the HDD? What about viruses in bios?

Answer 1

Viruses in the BIOS are still flowers.
Bad people can teach your computer to drink, smoke and swear - and nothing can be fixed, even by formatting the HDD.
update
In view of the fact that many people have already declared me a troll, I want to give an alternative answer.

1. I will make a reservation that it only applies to the so-called "standard PCs", on other architectures (eg ARM, MIPS), or with other bootloaders (eg Mac, Chromebook) it may be different.
   
2. At the moment, UEFI firmware is used for the initial boot of the computer . If you don’t feel like digging into the old trash with the BIOS - in this case, I refer you to specialists .
   
3. Today, there is an opportunity to modify UEFI , and bad people, trying to harm you, have probably mastered it.