Back up any important files to another hard drive, or a DVD, etc!

1.Insert the Mac OS 'Leopard' Install Disc and double-click the Install Mac OS 'Leopard' icon.


2. Follow the onscreen instructions. In the pane where you select the destination disk, select your current Mac OS X disk (your hard drive) (in most cases, it will be the only one available).


3.Click Options. If you want to save your existing files, users, and network settings, select “Archive and Install,” and then select “Preserve Users and Network Settings.” If you want to erase everything on your computer and reinstall Mac OS X, select “Erase and Install.” You can’t recover erased data.


4. Click Continue.


5. Click Customize to select which parts of Mac OS X you’d like to install, or click Install to perform a basic installation (recommended).

After this procedure, you'll find an 'old system' folder on your hard drive.
You can then manually transfer your old preference settings (software registrations, etc.) to your new system. NOTE: Be sure to put those old settings in the same location in your new system folder (usually the 'Library' folders).