One of the lame things about Time Machine is that backups over the network are only officially supported on Time Capsule. Luckily, even if you don't have one of HP's Time Machine-compatible Home Servers, you can make it work.
Brent Freidman lays out exactly how to get Time Machine backups to work with any Windows Home Server machine in a point-by-point method that's pretty easy, despite some Terminal tinkering to make it work—follow that here.
If you wanna use just a generic NAS, you can do that too. Personally, I followed this guide by Adam Cohen-Rose—though there are others—to get it going on a 4TB WD ShareSpace several months ago, though WD now has a much cheaper consumer version. It's a hack that also takes a little bit of work, but ditching the USB cable is totally worth it.
If you're in the market for a NAS (if you don't have one, you are, even if you don't know it yet) check out our guide to finding the perfect networked storage.