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What did you use to install it? I guess you were using some kind of control panel like Fantastico from your webspace?
If it fails to uninstall it ... well can´t help there.
For a manual uninstall:
1. Use phpMyAdmin to access your MySQL database
2. check all tables that start with xoops_ and then at the bottom select "Drop table" from the select box
3. Use an ftp programm to connect to your webspace and just delete all files in the XOOPS directory. (hard to tell how it is named by your install tool. However when you enter the directory and see subdirs like (cache, class, images, include, kernel, language, modules, templates_c ...) you are in the right place.
Finally why would you want to uninstall xoops?