Windows Home Server has no built-in mirroring. Windows Server 2008 R2 is the base operating system for Windows Home Server 2011; it
does have software mirroring built in. You can learn a great deal about how to use software RAID by consulting Technet, but to answer your specific question, no it is not possible to mirror just one partition; you must mirror the entire volume.
I don't recommend the use of software mirroring (or other software RAID levels), by the way. It's not particularly hard to set up, but it doesn't perform all that well, and recovering a degraded array isn't as simple as Microsoft's documentation would have
one believe.
I'm not on the WHS team, I just post a lot. :)