If you want to keep everything as it is, why don't you stay with XAMPP 1.6.6?
The recommend and best method to switch from XAMPP 1.6.6 to XAMPP 1.7.4 is to install a fresh version XAMPP 1.7.4, Only change something on the configuration files if it is really needed (e.g. a requirement for a script). Upgrade to XAMPP 1.7.4 isn't possible, because there is no upgrade package for XAMPP 1.7.1 to XAMPP 1.7.2 and from XAMPP 1.7.3 to XAMPP 1.7.4.
You can use two XAMPP versions side by side to move your scripts step by step to the new environment. Simply use the zip version, unzip it inside the root of your Harddisk (e.g. in a directory like c:\xampp174\).
Last hint: If you don't really XAMPP 1.7.4 you should use XAMPP 1.7.3. That version was much more stable and functional then the current release.
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