New SAP GUI puts GuiXT out of action; why?

I was using GuiXT correctly and then my company changed the SAP GUI to SAP GUI 640. Since then I have been getting the following error message:
Different versions of guixt.exe and guixt.dll
2005 Q3 4 Beta0 / 2005 Q4 1Beta0
Can you please explain why.

The GuiXT component consists of guixt.exe and guixt.dll. When GuiXT is started by sapgui, it checks that the file versions of guixt.exe and guixt.dll match, otherwise GuiXT stops and shows the error message
"Different versions of ...", the message that you just got.
In most cases, this occurs if the file guixt.dll is still "in use" when one installs a new version. We have never seen a situation in which the sapgui patch contains different versions of the guixt files.
I suggest that you install the most recent GuiXT version, 2006 Q3 1, running the setup program guixtsetup.exe that is available for download at
http://www.synactive.com. Or you can install a more recent patch of sapgui 6.40 (6.40 patch 20 is now delivered by SAP) that will also contain a 2006 version of GuiXT.