I don't know why but :
I'm upgrading XAMPP 1.6.7 to latest (1.7.3) - I use the xampp-win32-upgrade-1.7.2-1.7.3.exe file.
My installation is on D:\xampp
The upgrade was successfull but at the end there was a first error which said that the xampp_???.php file in intsall directory was missing.
Finaly, my web application was not working at all because MySQL module in php was not launged. (No indication in a phpinfo())
(Mysql & apache services were started & restarted)
If someone cal help me please !