If you find that you need to access a Mac from a Windows box, probably you’ll find yourself surfing the specs of a variety of Win32 X-Windows clients. Here are some that I tried:
Cygwin also has an X client, but I didn’t muck around with that.
RealVNC is (I think) the original. I’m guessing that the others are forks of Real, and that either Echo is a fork of Ultra or Ultra a fork of Echo. Tight seems to be a fork of an earlier, simpler version of RealVNC. And, according to their website, is “fully compatible with the standard RFB protocol used in VNC.”
It’s not surprising then that, out of the box, TightVNC is the only client I could get to connect to a Mac from my Vista machine at home. Tight’s refresh/rendering is plenty slow, but gets the job done.