For USB...

Smoothstepper is popular:
http://www.warp9td.com/

There's also CNCUSB:
www.planetcnc.com

LAN interfaces seem to generally be a lot more expensive, some examples here:

www.slidesandballscrews.com

I'm sure you'll find more.

To 'get away from Windows timing problems' you could just use EMC2:
www.linuxcnc.org

That doesn't entirely eliminate the 'problem', but do you need to? Clearly many people use the parallel port, so what are you making that requires a better interface or is it just for convenience?