i use
TightVNC in Windows to remote control my Mac running Leopard and
vice versa. EDIT: RDP doesnt do this.
if your running Leopard you dont need a VNC client as its built in but if your running Tiger use Chicken Of The VNC or something similar.
to do this install Tight VNC on the PC and set it up. its pretty self explanitory and opens the right ports in the Windows firewall.
alls you have to do in Leopard is tick "Screen Sharing" in Sharing Preferences, add a password in options and then in the Finder press cmd+K (or choose Connect to Server in the Go menu) and type "VNC:\\
the PC's IP address and input the PC's password.
then to control the Mac from the PC launch TightVNC server, type in your Mac's IP address and input the password you set before.
note: you will have to know the PC's and Macs local IP address to do this. a tip would be to get your router to give static IPs or assign IPs by MAC address so you know them.
P.S. there is a hint
here to give you more options for Leopard's VNC client for colours and privacy which might help you and also gives you a recents list so you dont have to use the Connect to Server method.