The transition is not necessarily easy, but not hard also. PHP 5.3 brings some changes to the internal Zend API, so some PHP extension need to be upgraded (I had to upgrade xDebug). That means that you need to find the respective DLLs, which may not be that easy, depending on your current setup.
The config file, php.ini, is pretty much the same. You will actually have to take some things out actually (for example extension=php_pdo.dll is not needed anymore).
Just execute php -m from command line and see what errors are thrown. I have just copy-pasted the php.ini file from a 5.2 release and was done configuring 5.3 in a couple of minutes.
I mentioned xDebug a few lines above. If you use it, you should know that the line which activates xDebug is now: