I assume you have the latest version of Mac OS X Tiger, 10.4.11? And you also have installed all available updates via Software Update?
And what exact Mac do you have?
And does it only happen in Finder or in other applications to? Does it go away when using Exposé or switching applications? Is it behind an active window or is it in front?
Have you tried this yet? Resetting your Mac's PRAM and NVRAM