New to Java

hello,

i'm a student newly introduced to the world of java and i'm trying to compile and run java files using the command prompt in windows xp. i have already installed jdk1.5.0_09 and use the following command to compile my java files...

javac -d . filename.java

this method taught to us works fine on my institute's lab pcs. however at home i get the following message...

'javac' is not recognized as an internal or external command, operable program or batch file.

can someone please tell me as to where i'm going wrong?