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Try un-installing the drivers that came with it, unplug the joypad, then reboot.
When the computer comes up and you have logged on, plug the joypad into a different USB port and let Vista do its thing and find a driver for it. *If a UAC prompt comes up, just click continue, if it doesn't then its still good*
Once Vista has found a driver, click the start orb and type game controllers then hit enter, when the window comes up, you should see something like "twin USB joystick" or something like that.
Double click it, and click the settings tab in the new window, then click calibrate and follow the steps in the wizard.
Once you have done that, you should be able to use the controller. Open the game that you was trying to play and you should now be able to play it!
If you have any problems, just post back!
Hope this helps,