It seems to be behaving normally, at least for now. Thank you so much for your help!
How will I know that it is 100% okay though? I'm sort of worried because the anti-virus and anti-malware programs didn't detect problems even when things weren't right, so I feel like I can't depend on them to know that this problem has been finally fully resolved.
Is reformatting the only way to be 100% sure that everything is now safe and secure again?