I have a Kensington Trackball (the original grey square 4 button model) which works great and which I absolutely love. I have had it about 10 years now and it still works perfectly fine. Kensington got a lot of grief (and rightly so) for not providing driver support when migrating it from...
buttonmapping
compatibility
compatibility mode
dell inspiron
device recognition
driver support
hardware issues
installation issues
kensington
legacy devices
mouseworks
software capabilities
technical support
trackball
troubleshooting
user experience
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windows 7
windows migration
windows vista