I am using PhpED on Window. I am developing my code against the PHP 5.3 RC1. This version has some new functions that do not exist in the previous version. Is there a way for me to have PhpED use my installation of PHP?

you can use any PHP, like the one that comes with TechPlatform, or with XAMPP or anyhow else. The only thing you'd have to do is to install debugger module, and it's what is still not available for php version 5.3-beta.