error for No valid Qt version set

[zhongxichuan]

I'm a new Qt4 lover,and my operation system is windows xp .
I had install the qtcreator-1.2.1,after I finished the hello_world program,
I complie it but the error happen:
hello_word.cpp as fellow:

#include <QApplication>
#include <QPushButton>
int main(int argc, char *argv[])
{
QApplication app(argc,argv);
QPushButton b("Hello World !");
b.show();
QObject::connect(&b,SIGNAL(clicked()),&app,SLOT(qu it()));
return app.exec();
}
and the complie output show :
No valid Qt version set. Set one in Tools/Options
Error while building project table
When executing build step 'QMake'
Canceled build.
Here,I turn to tools-options,then the Qt4-Versions show:
qtcreator-1.2.1 path is D:\Qt\qtcreator-1.2.1
version name :qtcreator-1.2.1
path :\Qt\qtcreator-1.2.1
but Debugging Helper is red ×
the bellow show:\Qt\qtcreator-1.2.1 is not a valid Qt directory
But you see,I installed in D:\Qt\qtcreator-1.2.1
Why the hello_world.cpp couldn't complie pass?
and how to reset the configure?
could you help me?thank you