you need to set the class path variable  CATALINA_HOME to the install  dirctory of
tomcat. the procees is diffrent for window and Linux.

In Win2K

My Computer => Properties => Advanced ==> Environment Variables

add: CATALINA_HOME = c:\Tomcat (assuming this is where you installed it)
add: JAVA_HOME = c:\jdk1.3 (ditto)

In Win98

autoexec.bat

add SET CATALINA_HOME = c:\Tomcat
add SET JAVA_HOME = c:\jdk1.3

check this out:

http://www.moreservlets.com/Using-Tomcat-4.html

I assume u r using Tomcat Integrated App and Web server

Set JAVA_HOME variable  in the classpath of ur machine
to point to default Java installed directory

U will not get this error  message again