I have pentium-4 desktop with windows 7.When I start my computer with a new dell monitor, optical mouse is not working.

Solution
The monitor would have nothing to do with the mouse not working the connections do not affect each other in any way. Need more details here, is this a dual monitor setup or a single monitor? Did you try a different mouse or a different port? Is the only thing you did was replace/add the monitor? Did you make sure the cable did not get pulled out?
The monitor would have nothing to do with the mouse not working the connections do not affect each other in any way. Need more details here, is this a dual monitor setup or a single monitor? Did you try a different mouse or a different port? Is the only thing you did was replace/add the monitor? Did you make sure the cable did not get pulled out?
 
Solution