WordPress MU

I'd like to install wordpress Mu this way:
blogs.mydomain.com
with blogs like blog1.mydomain.com, blog2.mydomain.com, but i want to use http://www.mydomain.com and mydomain.com for another thing.
So i'd like to put the signup page in blogs.mydomain.com for example, how can i do that?

You can't do that.

Install MU where the existing site is and integrate the two. (That is, make the existing site the main blog in MU).