I have had these kind of problems in the past but the system restore feature has come to my rescue.
regrding anti virus and MS defender I pay BT £10 a month for tech support which as I have a small business I feel is a good investment having helped with some tricky issue s in the past. BT bundle Macafee anti virus in with some of their Broadband packages but the tech support people said that they recommemded Defender instead if you are running Windows 10 as t did a very effective job and didnt interfere at all with Windows; so that seemed a pretty good endorsement. Mind you it is still a good idea to run a malware scan from time to time with one of the free Malware scan tools.