Can I build my own Windows Home Server?
Sure you can. When its commercially available, you can buy just the software and install it on your own hardware, or at least that's what Microsoft has announced at this point (Beta release notes and other conversations). While the OEM products from HP and other vendors are targeted at the less computer savvy, they realize there is a large enthusiast market who will want to build their own Windows Home Server on their own hardware (new, or just used bits laying around).
Robert McLaws, who titles himself "an online pundit who's barely old enough to legally buy alcohol" has put together a shopping list from NewEgg.com (http://www.newegg.com) that will allow you to build a respectable Window Home Server box for under $500. Of course, this doesn't include the software, as it hasn't been released yet.
Note: This does not include the Windows Home Server software, and the case Robert chose only has one internal 3.5" drive bay, and one 3.5" external drive bay. The external bay is usually used for a floppy or card reader. Having more than one hard disk allows Windows Home Server to utilize "Folder Duplication". This means that Windows Home Server will automatically create a duplicate copy of any Shared Folders where duplication is turned on. With a single hard disk, this isn't possible.