inconsistent behaviour of glscissors

Hello, I use Windows 7 and Windows XP, SDL+OpenGL, mingW c++. I am using glscissors
to do some dirty-rect animation, with the intention of getting very smooth 2D animation
on my older XP computer that supports OpenGL 1.3. I use


SDL_Rect aRect = Extra_life.Location();
glScissor(aRect.x, 480 - aRect.h - aRect.y, aRect.w, aRect.h);


with the result that everyting works fine on my win7 computer, but on my
winXP computer (supports OpenGL 1.3), it only works fine in windowed mode, not
fullscreen. In fullscreen, glClear() seems to forget that the rest of the screen isn’t inside
the scissored area, and thus only the background in the scissored area ends up being visible
(when I put it back on and put the image back on top of it).

The point is, I enable the scissors, then call gl_scisors(), then glClear(), but glClear affects the
area outside the scissored area. Possibly pertinent information is, that the fullscreen mode on WinXP is also Vsynced,
and I wonder if that has anything to do with this behaviour. Without Vsync, the windowed version
runs at about 150 fps without any attempts at scissoring, every loop, while the fullscreen mode is always 60 fps.

Thanks in advance for any and all responses!

Addendum 6-20-12 9:24AM EST: The windowed version automatically doesn’t vsync, while the fullscreen displays
60 fps (vsyncs), despite coding attempts to modify.

Addendum 6-20-12 11:51 AM EST: The fullscreen mode is now free of vsync, but still has the wrong scissor behaviour.