how do i handle the above situation ? how can i figure out through my software as to which is the default email client for a given user and then proceed to use either MAPI or OLE automation.

You cannot use automation with Outlook Express. You can go with a MAPI solution because MAPI will just use the MAPI compliant e-mail client installed on the local machine.

You do have other alternatives as well. Blat and wwwipstuff are two that come to mind....