You need to download and install the latest java runtime environment (JRE6 update 2). Try this link
http://www.java.com/en/download/manual.jsp. After you install JRE6, you will need to update the HEC-HMS command file. This file is named "HEC-HMS.cmd"
and is located on my computer in the following folder, C:\Program Files\HEC\HEC-HMS\3.1.0. Open this file using Notepad. The last line in this file should look like ----- "C:\ProgramFiles\Java\jre1.6.0_02\bin\javaw.exe" -Xcheck:jni -ms32M -mx1024M %PY_OPTS% %JAVA_LIB_OPT% -classpath %JARS% %PROG% %ARGS%. Yours will differ based on where you installed JRE6. Attached is a copy of a modified command file. You will have to start HEC-HMS by double clicking on the modified command file. You could create a shortcut to the command file and place that short cut icon on your desktop.