Har du innstallert JRE fra samme side du lastet ned JDK fra?
http://java.sun.com/j2se/1.5.0/download.jsp


Edit:
(The JDK also contains a private JRE for use only by its tools.)
# Private vs. public JRE - Installing the JDK installs a private J2SE Runtime Environment (JRE) and optionally a public copy. The private JRE is required to run the tools included with the JDK. It has no registry settings and is contained entirely in a jre directory (typically at CProgram Files\jdk1.5.0\jre) whose location is known only to the JDK. On the other hand, the public JRE can be used by other Java applications, is contained outside the JDK (typically at CProgram Files\Java\jre1.5.0), is registered with the Windows registry (at HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft), can be removed using Add/Remove Programs, might or might not be registered with browsers, and might or might not have java.exe copied to the Windows system directory (making it the default system Java platform or not).