Upgrading an old Windows install.

Greetings Ladies and Gents,
We've used PHP for many years on our old Windows 2000 server running IIS5. 
We currently have PHP 4.4.1 running, mostly because I am dumb and lazy and 
it's just always worked so we've kind of went with the "if it ain't broke, 
don't fix it" motto. Security wise, it's probably not been smart but anyway. 
We now need to get PHP 5 running. I'm wondering if anyone can give me a 
heads up on any known differences that may break any sites setup using the 
PHP 4.4.1? Is it recommended I just go with the latest? I've been doing some 
reading and it sounds like maybe I should use 5.2.9-1 instead of the latest 
5.3.6?

Any info., tips, tricks, etc... would be appreciated.

Thanks,
Scot