What can cause a full screen refresh ?

[vtgav]

I noticed that 'sometimes' refreshing my browser (IE6) causes the full PC screen to flash - presumably a screen refresh (Win XP). I finally tracked this down to some websites with javascript on them ( to be honest, it is a couple of my sites causing this) the code is simple stuff for rollovers etc.

Is there some sort of idiot mistake I may have made that could cause the whole screen to refresh ??

any suggestions or ideas appreciated,
thanks, Gav