Sort out Joomla Site Error

Odd things are happening to our Joomla site. When the site is visited, we get this message: 404 - Component not found You may not be able to visit this page because of: 1. an out-of-date bookmark/favourite 2. a search engine that has an out-of-date listing for this site 3. a mistyped address 4. you have no access to this page 5. The requested resource was not found. 6. An error has occurred while processing your request. When attempting to access the administrator area of Joomla, one gets: * Error loading Components: Incorrect key file for table 'jos_components'; try to repair it SQL=SELECT * FROM jos_components WHERE parent = 0 * Error loading Components: Incorrect key file for table 'jos_components'; try to repair it SQL=SELECT * FROM jos_components WHERE parent = 0 * Error loading Components: Incorrect key file for table 'jos_components'; try to repair it SQL=SELECT * FROM jos_components WHERE parent = 0 * Error loading Components: Incorrect key file for table 'jos_components'; try to repair it SQL=SELECT * FROM jos_components WHERE parent = 0 and Error loading Components: Incorrect key file for table 'jos_components'; try to repair it SQL=SELECT * FROM jos_components WHERE parent = 0 There is also, as we have AVG antivirus installed on our server, a exploit threat alert that comes up, but which we know to be unwarranted and the result of a piece of otherwise redundant code in Joomla that can throw up this alert. We need a joomla expert who understands the above and how to correct it.