Note that while I'm a Windows admin and have php running under IIS, I don't run Moodle in that environment. So here is how I would approach this issue:
1. Using the moodle migration instructions, I'd make a copy of my production site and upgrade that to 1.9.19+.
2. After verifying that the test upgrade is working, I would upgrade my production site to 1.9.19+. The reason for doing this first is that older versions of 1.9 do not support php 5.3, and there are a number of pre-2.x upgrade issues fixed in later 1.9 versions. The 1.9 upgrade should be relatively safe, but testing first is always encouraged.
3. Upgrade php to the latest 5.3 version. You'll find it here: http://windows.php.net/download/. You should probably first use control panel's "change" option to note the options that php 5.2 used, and save a copy of C:\Program Files\php\php.ini so you have a reference for getting the settings right in the new version. You may also need to know which web server software Moodle runs under, whether Apache or IIS as that may affect how php is installed.
4. Once php is upgraded and Moodle is still running successfully and shows php 5.3, then proceed to upgrade your test site from 1.9.19+ to 2.2.9+ to 2.4.3+. Note that the incremental upgrade steps are necessary for the upgrades to work properly, and you should test that your Moodle is still working after each one.
Always upgrade first on a copy of your production site. Always back up your production site fully before attempting an upgrade (moodle files, moodle data files, and database).