If you've been infected with malware, wipe your drive and start over. Reinstall Windows. If it's a rogue employee of a company you were calling, contact the company and let them know. Any general-purpose operating system is vulnerable to these kinds of malware attacks. If you positively need to use Windows at home, you sort of should become a guru of PC security to protect yourself. Windows shouldn't be your default OS pick anymore.
Don recently got a used PC, but it's locked with a password. Since the password prompt comes up after the PC has loaded Windows, Leo says it's easy to wipe it and reinstall the operating system. All he needs to do is download the Windows 10 installation tool from Microsoft called the "Media Creator's Tool", and put that on a thumb drive. Then he can boot to that drive, format, and reinstall it from there.
Lois has an HP computer that is running very slowly. She's wondering if there's any program she could get to speed it up. Leo says there are cleaners, but none of them are any good and she should avoid them. The only and best way to fix a slow computer is to start over fresh. Lois should backup her data and then run the recovery disk, wipe the drive, and reinstall Windows. Most computers nowadays don't come with recovery disks, but computers 5 years ago likely did come with them.
It might be possible, but it depends on how the tech did the reinstall. Whenever you delete a file from an operating system, it doesn't actually erase that data. Instead the files are simply renamed so the PC will recognize that as free space to use. So there's a chance the data is still there, and there are a number of free unerase programs. However, once the file allocation table (sort of like a hard drive's 'table of contents') has specified that data as free "unallocated" space, it can be written to. And once the data is overwritten, it can't be recovered.