Here's what I did after reading the other posts in this thread:

0. Backup or dump the contents of the database, copy your current drupal setup in your webdirectory to another folder, maybe call it www_bak . Realize that if you tried to install ubercart without cleaning out the ecommerce that you will need to also clean out ubercart records.

1. Print out the descriptions of your current products for reference later. You may want to reenter the products when ubercart is running and not have to rethink your descriptions. Also comes in handy when deleting records in _node_type later in this list. If you forget try using Google's cached pages of your website.

2. Be sure you moved your ubercart folder out of ../modules/ , that is, if you tried to install ubercart without vacuuming for ecommerce first.

3. Move your ecommerce folder out of ../modules/

4. Delete your ecommerce tables from your database that refer to any ecommerce module. Usually done through PHPMyAdmin.

5. Realize that you may have to review the _taxonomy table also. I didn't have to review the records in this table.

6. Empty any table with the name cache, this includes _cache_filter, _cache_menu, _cache_page. As Ryan says, use the Empty button if you're using PHPMyAdmin.

7. Remove records that are products in the table _node, this will be pretty clear. Remove records that are products in _node_type, this is not so clear so refer to your product printout in step 1. Eyeball the other _node tables for product entries.

8. Remove records in the table _system that mention ecommerce. If you installed ubercart then clean out those records also.

9. Install ubercart.

Hope that helps.