How do I repair my user db table in phpmyadmin when I get error message that is is corrupted?

When I try and edit user accounts in phpmyadmin I get the error message "#1034 index for table 'db' is corrupt". The only table 'db' I can see is in the 'mysql' DB. Firstly, is that the table it is referring to? Because when I browse it, it seems fine and I get no errors there. I am simply clicking the 'user accounts' button in phpmyadmin, at the top. That is how I normally edit users, not directly in a db table. But I cannot get past that. I am wondering whether I need to re-install xampp / apache.