"in my game,I have drawn maybe 400-500 objects.Will I have to adjust all of them?It will surely take a lot of time."
You absolutely will want to find a "general" solution that will take care of scaling your Window (or whatever XNA calls its display), and all the objects in it. I bet that can be found.
I think you probably can get some help on an XNA programmer's forum. 'DreamInCode seems to be a very active one. A quick search turned up this likely resource: "Creating Resolution Independence For PC:" [
^].