Thanks for the advice but since I posted this I got the message that the scan had been completed and no threats found. It seems that the information panel was not reflecting what was actually happening? My computer has now returned to its normal speed too as it was very slow whilst scanning.
My Windows is up to date as I use automatic updates (but have not braved Windows 11 yet!). My computer came with McAfee installed and all I had to do was activate it so not sure how to do a reinstall. I did say my computer experience was limited.
As the problem seems to have solved itself, I'll leave it for now.