how about the 'variables' table?

Its impossible. The variable table is name/value. There is no data saying were or what the variable is for. Your best bet is to save your database and go through it with phpMyAdmin. Or you can use this tool, see what modules you did install and then install then uninstall them. You can also look at the code and see if they remove all the variables. Never understood why they didn't have a module id.

I would suggest you take a close look at the data before you start playing with this tool. If you delete a module that has not been uninstalled properly and you try to install it later you could end up with a mess on your hands!