You are probably going to have to get into phpmyadmin to see if you can repair that table or replace it. Apparently the postindex table isn't that important but can be a pita. You can delete and recreate it in fact but I guess I would try to empty it first and rebuild your search index. If the table is corrupted, you may have to delete it and recreate it then run your update counter script.
Admin CP -> Import & Maintenance -> Update Counters -> Rebuild Search Index.