Looks like a GUI glitch. Do you get this in every app or just Mail and Finder?
And which version are you using? 10.5 or 10.5.1? If using 10.5.1, I would reinstall the 10.5.1 update but take big precautions with the install:
1 - go to Apple/Support/Downloads and spot the 10.5.1 updater for your Mac model
2 - download the installer
3 - unplug all unnecessary peripherals, only keep keyboard and mouse
4 - restart in Safe Boot mode by holding down the
Shift key
5 - install the update - don't use the Mac while the installation is underway
6 - restart normally.
See if that didn't fix the problem. Report back.
Good luck!