If it has a 'wavey' effect then your graphics card drivers need updating.
If it only affects some sites and you are using XP then it's the site you visit that have upgraded to Vista or Windows7e servers so you won't be able to do much about it.
If you have a desktop PC and this affects all sites then your graphics card (or 'onboard') is the problem. It's running on Windows generic drivers for some reason.
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