Open the Task Manager(Ctrl+Alt+Delete) and see if LW is listed as one of the Processes running and end the process. Then try installing it.

You shouldn't have to uninstall LW, by installing a new version it'll automatically uninstall the old one. If you do want to uninstall LW manually, go to C:\Program Files\LimeWire\uninstall.exe, and double click it.

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If it ain't broke, fix it till it is!