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Depends on what is wrong. Do you know if it is a file or database problem?
Anyway, for your information you can re-run install and it won't overwrite your existing tables--but I don't know if this will be an advantage to you if your database has problems. To run install upload your install folder and the original mainfile.php to your server.
If this does not work you will need to drop all your tables using phpadmin. You can also dump a backup of your database using phpadmin. In phpadmin you go to "Export", remember to "select all", and choose all the other options available like "insert" options.
After you you completely re-install XOOPS and a new database, and install your modules, you can use the "insert" data to salvage previous information for users and modules using phpadmin i.e.
INSERT INTO `xoops_users` etc etc.
This can be a bit tricky if you have not done it before though