After installing the arudino software on an XP machine. I execute the run batch file and I receive a "Java.lang.NumberFormatException:null" error. I have installed Java on a XP SP2 machine. Please help. I feel really incompetent as I have tried to install arudino using Linux and Windows XP and I am still out of luck. I am a high school teacher here in Canada and I really want to have my students work on the Arduino board. Please help!!! 
Which version of Arduino are you trying to install?
I am trying to install version 16 and version 17. Thanks for responding and looking into this
Sounds like this bug here:
http://code.google.com/p/arduino/issues/detail?id=79&can=1&colspec=ID%20Type%20Status%20Priority%20Component%20Milestone%20Owner%20Summary
Fix it by deleting the preferences.txt created by an earlier Arduino installation:
On linux its at "[UsersHomeDir]/.arduino/preferences.txt" (mind the dot in from of arduino).
On windows I don't know...
Eberhard
The solution was a clean install of XP. Then download the Java virtual machine from the sun site. Install arduino software and everything works well. Thanks for those who responded!!!!!!