@TehFedro: Hopefully what SeriouslyNow said will fix it. Another thing I just thought of is that it might be a hardware compatibility issue.
Sounds like what the other guy mentioned is definitely the problem though. As for downclocking, just grab the software and google up a guide, should be plenty out there if this issue is common =P Good luck!
@krystians: C# (pronounced C sharp) is an object oriented programming language, its very similar to Java. The main difference is that XNA runs off C#, and that is what you need to make games for XBL. Though you can easily program games in Java or C++ or C# if you wanted to, its just that Microsoft has a very nice setup for XNA.