You can run a relay in Windows following this tutorials:

• For running a guard relay in Windows, please read: http://community.torproject.i2p/relay/setup/guard/windows/
• For running a bridge relay in Windows, please read: http://community.torproject.i2p/relay/setup/bridge/windows/

You should only run a Windows relay if you can run it 24/7. If you are unable to guarantee that, Snowflake is a better way to contribute your resources to the Tor network.