Have you tried using VNC instead of X forwarding? I found that the difference between X forwarding over the internet compared with VNC over the internet to be significant improvement; that is to say, I can comfortably use VNC over the internet, but could not comfortably use apps with X forwarding. VNC (Tightvnc) is freely available for both Linux and Windows (server and client), so there's no interoperability issues, and over a LAN, speeds should be more than adequate.
I have only the ssh port open on my router, so have to forward ports using ssh before I can see my vncserver, but it isn't too much hassle. The best howto I've found on this is here: http://www.ibm.com/developerworks/li...AGX59&S_CMP=GR
(scroll down to trick 6). You can compress the data using jpeg compression, which degrades the image slightly, but is otherwise perfectly useable. You can also switch compression off, though things start to lag for me if I do (remember, I'm doing this over the internet, not LAN).
If you're firewall free, or if you poke holes in the Windows firewall (you should get a pop-up asking if you want to) and also in the Linux one (if you have one running), it should be as simple as, from the Linux box:
linuxserver@lan ~$ vncserver -geometry 1024x768 -depth 24 :99
and then from the Windows box
winclient@lan ~$ vncviewer linuxserver:99
I'm assuming you're using something like puTTY in Windows?...
edit: stupid me! Windows has a GUI client to connect with, so you'd open that, and enter either the IP followed by :99 (192.168.1.65:99), or if it works, the linuxserver's hostname followed by :99 (linuxserver:99).
To date, this is the most satisfactory solution I've found to accessing my computer at home from work graphically (as opposed to plain old terminal ssh).