If it is a spontaneous change in the system response, a bug check would be part of my response too.
Press CTRL, ALT, DEL to bring up the task list, and see if anything unwelcome is there slowing the machine. If so, kill it. Do not kill automatic updates or scans of security programs or other legitimate background tasks.
Make sure other users of the PC haven't changed the properties of the mouse on you, there is a slider that affects the device response speed and double click delay sensitivity in the Mouse applet in Control Panel.