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Are you using a wireless connection? Are you sure that there is not someone using your bandwidth in the neighborhood? If the router is not properly configured, someone can use your DSL connection to navigate Internet
Another possible cause is that currently servers are so busy that the connection is slow for everyone.
Another possible cause is that you are downloading something else with emule or torrent maybe?
For unregistered users, moreover, rapidshare purposely reduces the speed allowed in the downloads