u can set it through the my computer properties, right click my computer icon on ur desktop, click the "Advanced" tab, click "Enviroment Variables"
under the "system variables" area click "New", type "CLASSPATH" in the name area and in the value area, here's wat 2 do
now u av to copy the path where java was installed on ur compter containing the tools.jar file, for example if java was installed on ".;C:\Program Files\Java\JDK1.6_02\lib\tools.jar"
this will be d directory u have to go and set in the value area
after dis click okay, be sure to add the ".;" sign b4 d value now scroll down in the same system variable are and click the variable labeled "PATH", now here u have to go to the end of all d values and add the path where bin directory is in the JRE folder
for example ";C:\Program Files\Java\JRE1.6_02\bin" and copy it at the end of all the values be sure to add the ";" sign before all the values, after dat click okay and dat's all
so u're up and running java programms on ur system