Okay, let me think about it...
I would do something like this:
1. see Point 1 at JonBs post. very very important: Better do one backup too much than one to less. It's better to be safe than sorry! Good advise is: Set a Windows restore point or make a complete HDD Image if you have some Image program like Acronis True Image or Norton Ghost.
2. if you have some big databases (hundreds of tables, hundreds of thousant of entrys), don't use phpMyAdmin for exporting your databases (you could ran into an timeout without any error message and get an uncomplete/corrupt export file). Better use a script which avoids the timeout (i use mysqldumper
for that), the command line tool mysqldump, or just use your local copy of the xampp\mysql\data folder for that. Last option is just a good idea if you don't jump too many releases at one step or backwards).
Make some notes about the existing MySQL Users and Permissions! You will need to restore the Application Users later.
If you do DB-Exports, some DB tables like phpmyadmin or mysql are not needed. Just Export the Databases of your own Applications
3. i would simply move the whole old xampp folder somewhere. Thats especially a good idea if you want to use the copy mysql data folder option. But before you do that, be sure that all xampp services are unregistered, all xampp components are stopped and all xampp applications (like the xampp control panel) are closed
4. Install a fresh copy of XAMPP 1.7.3. Follow the default installation instructions for that => extract it into the root directory of your HDD, execute the setup_xampp.bat script
5. Don't compare the new version with the old one. Your fresh installation have some changes in the config files which are good practice or completely new. If you have later some problems, you can enable missing modules or change one or another configuration by hand. But that way you know what you have changed and you can inform yourself, why this change is needed and what this change will do.
6a. If you do the mysql data option: Copy the old xampp\mysql\data to your new xampp\mysql folder. If you were ask for overwriting something, deny.
6b. If you do the mysqldump command line export option, start the command line and restore the databases
6c. If you do the mysqldumper export option, restore it again with the mysqldumper script. Thats very self explaining.
6d. If you do the phpmyadmin export option, start the Apache and MySQL of your new XAMPP installation, login to phpmyadmin and import your export file
7. insert all needed mysql user (for example with phpmyadmin, you have to start your new Apache and MySQL for that)
8. Copy all your scripts from your htdocs folder to the new htdocs folder
9. Test your scripts.