You said if I install Firefox the problem will be solved, I've tried but it didn't work. Silverlight is a new software, there may be problems. Do you think this is a disadvantage?
No I think you misread what I was saying. You have a xaml file to create your silverlight app. It is giving you an error and I suggested it might be that the xaml file is badly formed. xaml is a form of xml and so it might be incorrect somewhere - a tag not closed, a closing tag with different casing and so on. I suggested you open the xaml file in firefox because firefox normally identifies the exact line of an error in xml files. Any xml parser would do. I was not suggesting that installing firefox would fix the problem. I haven't really worked with silverlight so I'm a bit surprised Visual Studio wouldn't give a more descriptive error.