GuiXT version problem: I get the error message "Different versions of guixt.exe and guixt.dll"

I suggest you use the Windows "Find" to locate all guixt.exe and guixt.dll files on your PC and to check their version (right mouse click on file name in MS Explorer, then click on "Attributes" and "Product Version"). Please observe that sapgui changed its default installation directory between 4.6D and 6.10. 
A  common problem when you try to copy guixt.exe and guixt.dll into the sapgui directory (sapgui 6.20): if saplogon is still active, guixt.dll is "in use" and is not copied.  In this case guixt.exe is new, but guixt.dll is still the old version. Please close all SAP sessions, open saplogon and close it again before copying.