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To start with, I take no responsibilty for any outcome in this. Just my two cents...
Given your initial post if I wanted to reinstall, I would (keeping the same version, of course - change versions and you're %$&#@%):
1) Make a backup of your database (using phpmyadmin you should be able to include drop statements) and your files;
2) Without deleting the mainfile.php, get rid of all your other old xoops core files;
3) Reload the core files to the version you were using without overwriting your mainfile.php;
4) Go into phpmyadmin (which hopefully you use as your database manager) and delete all the data and tables;
5) Run your previously saved database backup (which should be an sql query) via the phpmyadmin sql dialog box;
6) You should be finished.
I have done this many, many times for various reasons. You're facility with phpmyadmin is a key determinate in how well this works for you. Sorry, but due to other time commitments, I can't offer further help.
MMM...It tastes like chicken! 
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