For anyone interested, Kendall said in chat that:

1. Problem is only in chat.
2. The display runs off the right of the screen on his monitor.
3. Buttons at the bottom of the chat screen and the input box both disappear (or are off-screen).

My most recent guess is that updates to WinXP changed the meanings of some of the security levels in his IE, so even though he's got the same selections, they apply more restrictive levels of permissions than with his old setup. Since chat seems to be entirely in jscript, and that's blocked with some default security levels, it may be a matter of scripting being blocked. (add lots of ?????...?)

John