SetPixel not working on Vista

Hi,

SetPixel on Vista is acting strange. Certain areas it cannot draw on.
If I draw the entire window, then we can see the areas it can draw to,
which is vertical areas/bars that are a few hundred pixels wide. The
code works fine on my XP PC. So I thought there might be some window
app behind it preventing it from drawing in certain areas, but found
nothing. Changing the desktop to a solid color did nothing. Any ideas
what could be causing this? Here's the code,

void CTest1View::OnPaint()
{
    CPaintDC dc(this); // device context for painting
    if(IsIconic()){
        return;
    }
    long x,y;
    for(y=0; y < 1024; y++) {
        for(x=0; x < 1024; x++) {
            dc.SetPixel(x,y,0xff);
        }
    }
}

and another example,

void CTest2View::OnPaint()
{
    CPaintDC dc(this); // device context for painting
    if(IsIconic()){
        return;
    }
    long x,y;
    PAINTSTRUCT ps;
    BeginPaint(&ps);
    for(y=0; y < 1024; y++) {
        for(x=0; x < 1024; x++) {
            dc.SetPixel(x,y,0xff);
        }
    }
    EndPaint(&ps);
}

Also I tried dc,
int save = dc.SaveDC(); // place this near start of code
....
dc.RestoreDC(save); // place this near end of code

Also I tried calling OnDraw the common way within OnPaint-- same
results.

I've tried everything I know. Change the window size. Regardless, the
above code always draws vertical bars that are black, white, black,
white, about ~ 200 pixels per bar.

Thanks for any suggestions.
Paul