virus scan locks at 75% and goes no further have tried twice have left pc on for half an hour but still sticks
Which virus scan are you talking about? If Full Scan that can take a lot longer than 30mins depending on disk size and number of files to scan. It is not uncommon to stick a a percentage for a while before moving on. I suggest you give it more time a see what happens.
Second, I would try a repair of Avast.
It would help us all here if you listed your version of Avast and what Operating system you have.
OSis Windows 10 and Avast Premium Security is Virus guard