If you are using Windows XP you can uninstall by first making sure that Photoshop is not running.  Then go to the Start menu and select Control Panels.  Launch the Add or Remove Programs control panel, pick the name of the plug-in and then click on the Remove button.

For Windows Vista you want to go to the Window (Start) menu, select Control panel, and under the Programs option select Uninstall a Program.  Select the name of the plug-in and then click on the Uninstall button.

NOTE: Please make sure to deactivate the product is you are moving the plug-in to a new computer.