utorrent is a good client for windows users and it supports encryption as does azureus in case you have an ass hole ISP. from the faq:
Does µTorrent support Protocol Encryption (PE)?
Yes, as of version 1.4.1 beta build 407. It is compatible with Azureus 2.4.0.0 and BitComet 0.63.
Protocol Encryption (PE) is a joint specification between Azureus and µTorrent. It is designed to bypass throttling and/or blocking of BitTorrent traffic by an ISP.
You can choose Protocol Encryption's mode of operation in BitTorrent. Here is an explanation of the various options you can choose from:
* Disabled: Does not encrypt outgoing connections, but will accept encrypted incoming connections.
* Enabled: Attempts to encrypt outgoing connections, but will fall back to an unencrypted mode if the connection fails.
* Force: Attempts to encrypt outgoing connections, and will NOT fall back to an unencrypted mode if the connection fails.
* Allow legacy incoming connections enables or disables incoming legacy (non-encrypted) connections.
All modes will accept incoming encrypted connections (and the encryption is 2-way)!
http://www.utorrent.com