You can download BitTorrent by going to http://www.bittorrent.com/. This is based on the uTorrent code. uTorrent is smaller and is available here: http://www.utorrent.com/
There are many BitTorrent clients available, each with a large number of supporters. Saying that one is the best would serve only to spark debate. A few of the more popular ones are linked to below.
You can use several clients that are known to work for Vista Home Premium: uTorrent: Fast, small, efficient. Light on features. Azureus: All-in-one, powerful, but can be slow on older computers. A...
Some test`s online mean that the Bitcomet client is the fastest but some people say that its the same on all clients. Its only the amount of seeds that have something to say.
On Windows, the best client is probably Pidgin. On Linux, Kopete is actually slightly better than Pidgin. iChat (included with OS X) and Adium are both popular chat clients with Jabber / XMPP support...