You say IntelliMouse. If it is an optical Intellimouse, it may be part of a bad batch from Microsoft.
http://www.microsoft.com/hardware/mouse/letter.asp
My IntelliMouse died a few weeks ago as described in the link, and Microsoft said it would replace it. They said that the serial number was not part of the bad lot, but they'd replace it as under warranty.
It would be nice if Microsoft would not bury these notices deep in the support section of its web site, and with obtuse, haiku-length descriptors :-\