Welcome to JavaRanch!
On Sun's Java SE download page, there's a small link for
Previous Releases, where you will find Java 5.0.
You will see a number of options, and you probably want
JDK 5.0 Update 17 (no EE and no NetBeans). The JDK (Java Development Kit) includes the API classes.
Be sure to carefully follow the installation instructions for your operating system -- especially the part about updating your PATH variable.
Post back if you have any problems.