How to enable OS X screen sharing (VNC) through SSH?
Source: http://technotes.twosmallcoins.com/?p=279 (archived)
Putty is a good SSH client for Windows.

SSH into your remote OS X machine with an administrator’s log in and password.

Enable Remote Desktop (a.k.a. Screen Sharing, a.k.a. VNC) with this command:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart \
-activate -configure -access -on \
-clientopts -setvnclegacy -vnclegacy yes \
-clientopts -setvncpw -vncpw mypasswd \
-restart -agent -privs -all


Login using a VNC client. As I mentioned, TightVNC worked for me;
for some reason, RealVNC and UltraVNC didn’t.  Your password is
“mypasswd” (see the -vncpw flag in the above command;  you can and
should change this).

When you are done, turn off screen-sharing using your SSH session:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart \
-deactivate -configure -access -off