The concept of a BT tracker does not work if you disconnect shortly after finishing the download... It is a good idea to at least get a share ratio (upload / download) of 1.0 before disconnecting.
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On another note, there is an incredibly efficient new BT client (sadly win32 only) around, utorrent. It is only a 105 kb download and typically uses less than 6 MB of ram. It is a great option for dialup users that might not want to download a hugely sized client such as Azureus, or those that don't want to use 80+ MB of ram just to download a SE4 patch.
http://www.utorrent.com/