Use of Garbage Collection?

Question

Hi, I want to know what action is performed when we call Dispose() method. Is Object frees all resources quickly on Dispose() call or Dispose() marks the Object is ready for garbage collection. And What happened when we set Object reference to NULL. Actually I have Windows form application in .NET 2.0. And I want to call garbage collector after certain time is passed(For Example after 5 mins) to collect all unreferenced Object.