heres a good plug in if you can't get solid inspector to work. just make a group, right click then fix solid. i've had best results selecting no and no on the settings. run it to create your solid then explode the group. obviously pm if you get stuck
http://rhin.crai.archi.fr/rld/plugin_details.php?id=793
cottrell: are you still importing as .dwg? you really need to try and use the above and get it so you can import .stl. it will save you so much time. what are you stuck on with the slow speed? using the pictures above, you can put - (minus) what ever number you want...
i'd rather you and ricme ask for help so we can get the whole .stl and solidworks working for you