Actually, now it just seems to be XP SP3 on my laptop only. On my desktop, the problem hasn't happened since upgrading. Not sure what makes my laptop config unique. I fully uninstalled and reinstalled the ToolTipFixer, but the problem remained.
However, it looks like this may not really be an issue now since apparently the tooltip bug may no longer exist after SP3.
On a SP2 system, I can reproduce the tooltip focus problem easily - for example, just by going into Start, then All Programs, then sorting by name. But on SP3, I cannot make it happen anymore.
I went through the release notes for SP3 and didn't see anything obvious that might be related to fixing this problem, but it sure does seem to be fixed anyway. I guess only time will confirm this though.
Thanks for responding!