That error says that there is a problem with the "guest" table, and that it needs to be repaired. If you have access to your database through phpMyAdmin, here's a page that describes how to repair a table:
http://php.about.com/od/mysqladminis...pair_mysql.htm
If that doesn't work you might be able to truncate the table, but vbulletin has no table named "guest" so I have to assume that's a mod, and so I don't what the affect would be.