Buggy display of pop-up dialogs

When the mouse goes on a symbol, an e-mail, a task, a date or anything else which producess pop-up dialogs, the displayed dialog is buggy. It has wrong colors so that information are unreadable. In detail: the displayed dialog has random colors and symbols of my desktop which are mixed up without sense.

Ubuntu 8.10
Kernel 2.6.27-11-generic

ProblemType: Bug
Architecture: i386
Dependencies:

DistroRelease: Ubuntu 8.10
NonfreeKernelModules: nvidia
Package: mozilla-thunderbird None [modified: /var/lib/dpkg/info/mozilla-thunderbird.list]
PackageArchitecture: all
ProcEnviron:
 PATH=/home/username/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: thunderbird
Uname: Linux 2.6.27-11-generic i686