I have a home server that I built originally with the intention of just recording TV. But I've recently realized there's a great deal of cool things that can be done with your own always-on server. I picked up a new WD1500EARS drive and put it in my server with the intention of having it serve as, not only more media storage space, but also a networked backup location for my main PC and laptop. I also figured I could set aside some space to use as a Time Machine backup location for my hackintosh. I won't go into detail describing how to do this, instead I'll just point you to this excellent guide by Hupio:
http://hupio.wordpress.com/2008/04/27/osx-timemachine-and-sambawindows-share/
No other guide I found on the web mentions the critical step at the end, that you have to create a sparsebundle and put it on the shared folder for it to work.
A problem that may pop up is that Windows 7 is not really setup to act as a file server. The result is that OSX will disconnect from Windows due to some memory allocation error. This can be fixed with a couple of registry edits, described here:
http://alan.lamielle.net/2009/09/03/windows-7-nonpaged-pool-srv-error-2017
