The windows handle limit for your application is 10,000 handles. You're getting the error because your program is creating too many handles. You'll need to find the memory leak. As other users have suggested, use a Memory Profiler. I use the .Net Memory Profiler as well. Also, make sure you're calling the dispose method on controls if you're removing them from a form before the form closes (otherwise the controls won't dispose). You'll also have to make sure that there are no events registered with the control. I myself have the same issue, and despite what I already know, I still have some memory leaks that continue to elude me..