need some help.

mum updated avast then after restart it wouldnt start up. Ive done a refresh and now got it to turn on but im getting access denied on trying to open C:\
I have tried adding a new user and giving them admin rights but they also cannot gain access. Ive managed to get the built in admin account showing but im getting password wrong even though it doesnt give me a choice to put in a password.

Looking in the security tab i have no permissions to c drive but i also cannot give the permissions as it fails. I can however get into some folders that are on there including programs folder and windows as well as my documents.

It looks like i have lost admin permissions as i dont seem to be able to change permissions.

Any solution to this?