The link below has a list of problems associated with the Java install. Look through them and see if any of them relate to your problem.
http://www.java.com/en/download/help/index_installing.xml?user_os=Windows%20XP&user_jre=6.0
If that doesn't help, look up the "System Restore" tool for the version of Windows you're running and roll it back to a date when it was working.
Answered Jul 04, 2012
Edited Jul 04, 2012