r/Malwarebytes

I was just minding my own business on my pc when suddenly my cpu usage went high, and malwarebytes was using 100% disk in task manager. So, i opened malwarebytes and for some reason it was scanning my file system. However i did not do anything to initiate this. I finished the free trial weeks ago and i dont have the premium version, just the free version. Windows defender is my main antivirus too so im not sure as to what activated this scan. Is this normal?

To make a very very long story short, a family members business pc was infected with a virus. We took the pc in to a computer store and had them remove the virus. The guy who (I'm hoping) removed the virus then uploaded 'trend micro maximum security' onto the pc. (never heard of this anti-virus software tbh so idk how good it is).

Ever since we took it to the individual who supposedly removed the virus from the pc (he was very unprofessional and didn't communicate at all on what he was doing/what he found/removed from the pc) so the owner of the pc has been concerned and worried about turning the pc on again (and hooking it back up to the internet).

In essence, despite paying for the virus removal, the owner of the pc doesn't trust that the virus(s) were actually removed. I'm trying to get the owner of the pc to take action over this situation, because the virus was placed there by a hacker who had/has infiltrated the email and several accounts of the owner of the pc.

Anyway, enough with the backstory. (I felt it relevant to share the gist of it since this situation is both time sensitive and stressful to deal with).

The plan right now is, I'm going to download Malwarebytes onto a thumb drive so I can then help the owner of the pc by plugging it into the pc without requiring they download it via internet. Main question I have is in the title: can the full extent of the software be used without internet to be entirely certain there's no more virus on their system?

(Also, if I plug this thumb drive into their pc, will the thumb drive become infected too if there does happen to be virus' in the pc?)

Solutions that have been offered that sadly cannot be done:
X Reinstalling the OS

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Bonjour, je vous contacte car j'ai été récemment victime d'un piratage de mes données pas un programme malveillant, en installant des dlc du jeu train sim world 7, au démarrage qui a suivi cela, mon ordinateur s'est mis à freeze pendant 2 minutes, et après, c'est revenu à la normale, je me suis, après le coup, rendu compte que je venais de me faire siphonner mes données, mes mots de passes se trouvent désormais sur internet. J'ai téléchargé malwarebytes, effectué un scan total et il a éradiqué 6 menaces, depuis, le logiciel lua tools est désinstallé et j'ai viré les dlc, j'ai depuis changer les mots de passe de mes comptes discord, reddit, meta, google, Microsoft, amazon, bref, les plus importants, voici donc ma situation. Je tiens à préciser que chaque compte que j'utilise possède la connexion via la double authentification activée. Merci d'avance à tous ceux qui répondront à ce message, sachez que j'ai su les fuites par l'empreinte numérique de malwarebytes.