hosting - getting started - 3rd level domains

newbie questions ahead....

Am looking at moving a site currently hosted by Verio on to a box here in order to host an app. Box is running XP pro. Lets say my registered domain is mystore.com. I need to create several (couple dozen ?) third level domains, such as:

acme1.mystore.com
acme2.mystore.com

Are there limits to the # of third level domains to create?
How is this done w/ IIS?
How does IIS resolve a URL request (http://acme2.mystore.com/products.htm for example) to a directory on the box?

Need to use SSL in order to take CC orders. If orders are finalized at "secure.mystore.com", can i use just one certificate and "register" it at this 3rd level (secure) or do i register it at www.mystore.com ?

 > Are there limits to the # of third level domains to create?

This is only limited by your DNS provider.

 > How is this done w/ IIS?

It's not.

 > How does IIS resolve a URL request (http://acme2.mystore.com/products.htm
for example) to a directory on the box?

By using host headers, or by creating a site at each address.

 > Need to use SSL in order to take CC orders. If orders are finalized at
"secure.mystore.com", can i use just one certificate and "register" it at
this 3rd level (secure) or do i register it at <a style='text-decoration: underline;' href="http://www.mystore.com" target="_blank">www.mystore.com</a> ?

SSL certificates should be registered for the most specific common name
possible, to avoid errors. If you're using secure.mystore.com, then that's
the name that should be on the cert.<!-- ~MESSAGE_AFTER~ -->