Last I checked, Windows can share an internet connection without any third-party software...
That being said, it's not surprising that the connection is slow/buggy. PC's weren't made to be wireless access points. Anyway, you might want to try setting it up through Windows directly. You might get different results.
What kind of speeds are you getting? Use the speedtest.net app: http://itunes.apple.com/us/app/speedtest-net-mobile-speed/id300704847?mt=8
Edited by keyboardNinja, 13 July 2011 - 10:16 PM.