Actually Windows XP, 2000 Server and 2003 Server also support Remote desktop
(or Terminal Service) via HTTP - Very fast and with decent 128bit
encryption, I might add ;o)
This is also obtained through a web-page.
(Found a screenshot at the bottom of this page:
http://www.tzo.com/MainPageSupport/HowToPage/HowToXPRemoteDesktop.html)

/Martin

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VNC also provides a Java-based version, so you don't need any Software for
the client at all. By using the java-based version you are not in need of
the webpage if this is the only reason. 
Just use your favourite browser on the specified port of the server and log
in. By the way www.tightvnc.com offers some nice features like
jpeg-compression, filetransfer etc and is also realvnc compatible