Another "Can Drupal do this" question!
Okay ... actually, I'm pretty much convinced that this awesome CMS can do anything. I just need to know how; or more precisely, to know where to look to find out how. There're a lot of modules and a lot of pages of documentation to swim through, and I'm pretty sure I can figure it out but a nudge in the right direction would be appreciated.
Here's what I'm looking at doing (that I don't already have an idea of how it'd be done).
First, I'd like to use Drupal to create multiple sites, in separate directories off of one domain, sharing the same login information. I've heard Drupal is supposed to be able to do this, but I don't know how or which modules are involved. Would it actually involve separate Drupal installs, or just one mega-site that functioned as different sites?
Second, I'm not sure I want the inside parts of the site to look the same as the "front page." They should have either a separate theme, or a different CSS sheet, or just not have the title banner (even if I have to use a separate theme in order to do that). What's the best way to implement this? Would I have to use separate sites? (Note that that's not the only reason I want to have multiple Drupal installs, or at least multiple functional sites running off the same install. As long as I'm asking about that, though, I may as well ask if it'd work for this.)
And third, I was inspired by an idea from this post on Creating Passionate Users. She set up a system where the user was asked a question right off the bat, and based on that question changed the titles / headlines of the next screens.
I want to do something like that ... have the very first thing that the user sees be some kind of web form, that depending on the choice they pick the theme of the site is somewhat different. With, say, a different banner image, or maybe a different CSS sheet. Assuming that I can code the simple form, how would I go about setting it up to work with a Drupal install? Would I need to create the Drupal site in a subdirectory, and then use index.html for the form itself? And then have it read off a cookie in order to let the user bypass it on later visits, assuming they don't just bookmark the main site?
Finally, once this form is set up, how would I implement the theme switcher? Is there a module in Drupal to do that?
Many thanks in advance!

=-=
First = Drupal can host multiple sites off the same code base. I use subdomains to run more than one drupal site off of the same code base. Sharing users/sessions and other tables are done using the settings.php file. There is an array in there that would need to be set. I learned how to do this using: http://www.slideshare.net/buddaboy/drupal-multisites and similar found using google.
Second = you can style Drupal any way you see fit, see the theme developing guide and play with an already existing contrib theme to get a better understanding of what you can do. ie: want a totally different front page? create a page-front.tpl.php file or use a condition statement in your current page.tpl.php file
Three = Outside of my scope
There is a theme switcher module in the downloads area.
Cool! I suspected as much;
Cool! I suspected as much; and it looks like I have a lot of learning to do here.
I'd like to create a custom form that will interact with the theme switcher module, and that'll be the first thing that anyone sees ... maybe that question would be more appropriate for a different forum, though. Many thanks either way!
=-=
to do what you ask will require a custom module to interact with themeswitcher (called switchtheme.module). switchtheme works in a block.
http://drupal.org/project/switchtheme