[Javascript] Screen width/height issue with IE7

[rich gray]

I have reset my display settings to 1280x1024 px and I have this 
javascript code in a body onload event:

alert(screen.width+' '+screen.availWidth+' '+screen.height+' '+screen.availHeight);

Firefox/Opera/Chrome all report => 1280 1280 1024 986

IE7 reports => 1024 1024 819 789

Can anyone tell me why IE7 reports different values to the other browsers?

TIA
rich