"Why do they split the HD? Is there any benefit?"
-> It is likely just one hard drive, divided into two logical partitions--one for your use and the other containing a backup of the OS (but not your personal files and applications).

"Need advice as to whether Windows Live is better than Yahoo?"
-> That is purely a personal preference. (Eg: Which is better, apple pie or cherry pie?) Try them both and see which you prefer.

"Can I transfer all my e-mails I have in Yahoo to Windows Mail? "
-> No, that is not possible.

"Could I run two e-mail services?"
Yes, you can run as many as you wish.