I'm no expert, Tilly2, but I have found that, once a computer has started to do something, it's best to let it get on with whatever it's doing. A computer can be a moody thing when it isn't scanning, so don't upset it.
Well I guess you could change your scan settings so it only scans when you're not using the computer, or upgrade the computer to one that's capable of running a scan and being used at the same time.
Although I suspect this is posted more as a chatterbank post and not a technical post.
I sympathise. I'm running scans (taking all day) because my PC is suddenly responding as if it's in treacle or something. There ought to be an app that can monitor and inform you what resource is being used up or what app is causing issues or what hardware is failing or whatever :-(