You will need:
Windows Home Server (Or an SMB network share) with free space as big as your mac's HDD used space
Apple computer running 10.4 or 10.5
Network Connection between the two
I have not tried this with 10.4 Tiger, but i know its much easier than Leopard.
Step 1: Enable SAMBA backup on the Mac
Step 1: Open Terminal
Step 2: Type the command:
"sudo defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1"
with no quotes. Then type in your password.
You have enabled Time Machine SAMBA backup.
Goto Step 2 for Leopard, or skip step 2 if you have Tiger