I could reproduce this particular issue by following the below steps

1) Uninstall all java from the machine
2) Install Java 1.5.0_16 (build 02)
3) Open Java Control Panel
4) Go to the the Update tab and click Update Now button.
5) Update ballon comes up in the system tray, click on that
6) Java Update window comes up, scroll up-down the scrollbar of the textarea in that window.

If the text inside the textarea get overlappled (jumbled), this bug is reproducible.

I have tested it on Windows XP machine. This is reproducible with Java 1.5.0_17 even.