Modeless dialog box in a thread

Hi,

 I have a multi-threaded application. In the application there is a class
CDiskSpaceWarn corresponding to a modeless dialog box that I need to create
in DiskSpaceCheck thread which is spawned in the Application main dialog's
Oninitdialog method.

Here is the definition of the thread:

        UINT ThreadDiskSpaceWarn(LPVOID pvoid)
{
    extern CDiskSpaceWarn *pdlg;
    long unsigned int timer=0;
    ULARGE_INTEGER ll,pp,oo;
    pdlg = new CDiskSpaceWarn;

    GetDiskFreeSpaceEx(NULL,&ll,&pp,&oo);
    float fDiskNum= (static_cast<float>(static_cast<__int64>(oo.QuadPart /(1024
* 1024))))/1000;

    for(int i=0;1==1;++i)
    {
                if(fDiskNum<20.0)
                {
                      pdlg->Create(IDD_DISKSPACEWARN,AfxGetApp()->m_pMainWnd);
                    pdlg->ShowWindow(SW_SHOW);
                    pdlg->CenterWindow(NULL);
                    pdlg->RunModalLoop();
                }
            Sleep(3600000);
            if(!threadDiskSpaceWarn) //Closing?
                break;
    }

    return(0);
}

The EndDialog menthod for this modeless dialog is overridden as follows:

void CDiskSpaceWarn::OnOK()
{
    // TODO: Add extra validation here
    pdlg->EndModalLoop(100);

    pdlg->DestroyWindow();

    CDialog::OnOK();
}

The thread is doing nothing but checking the hard diskps space every one hr
and brings up a modeless dialogbox incase the diskspace is less than 20GB. It
works well in the Release mode. But it throws an assertion error in Debug
mode. I read somewhere that it is not a right way to use Main windows handle
in worker threads. Could you please tell me how to go about it.?

I need to fix this asap.
Thanks