Directory0: We are using the cache option in the "guixt.ini" file:
Replication   Yes                  
Directory0   C:\GuiXT\Temp

In some PC's we are now getting an error "Could not write to file...".
How can we provide write permission to the C:\GuiXT folder
?

This is not possible by means of GuiXT. But you do not need to specify Directory0. If it is not specified, GuiXT will use the Windows temp folder (environment variable TEMP or TMP) to create the cache folder.